It doesn't let me select tags.

Hello

usually this issue is that something is blocking the AJAX request to load (to query) the tags

if you can examine the network tab of your browser to find (search) for "core.php"
and examine the response headers and the response itself then you can copy the info here

otherwise i will need access to the website to be able to see the reason !
(you can send access via a PM)

I find that :
url: '/administrator/components/com_flexicontent/tasks/core.php?225bda64c3cbff5614ca841918448a4d=1',

Hello

yes that is the URL

but you need to go and find 'core.php' in network TAB
and see the response from server

in the network TAB of your browser, there are responses for HTML, CSS, JS, images, other AJAX requests , etc

I need the
1. - HTTP headers of the response for this URL
2. - and if there is a response i would like to see what is its text

I hope it's the correct information. I'm not an expert !

1 - Filter on : /administrator/index.php?option=com_flexicontent&task=items.add

{"En-têtes de la réponse (0 o)":{"headers":[{"name":"Cache-Control","value":"no-store, no-cache, must-revalidate, post-check=0, pre-check=0"},{"name":"Content-Encoding","value":"gzip"},{"name":"Content-Type","value":"text/html; charset=utf-8"},{"name":"Date","value":"Wed, 06 May 2020 15:11:43 GMT"},{"name":"Expires","value":"Wed, 17 Aug 2005 00:00:00 GMT"},{"name":"Last-Modified","value":"Wed, 06 May 2020 15:11:44 GMT"},{"name":"P3P","value":"CP=\"NOI ADM DEV PSAi COM NAV OUR OTRo STP IND DEM\""},{"name":"Pragma","value":"no-cache"},{"name":"Server","value":"Apache/2.4.25 (Debian)"},{"name":"Transfer-Encoding","value":"chunked"},{"name":"Vary","value":"Accept-Encoding"},{"name":"X-Content-Encoded-By","value":"Joomla"},{"name":"X-Powered-By","value":"PHP/7.2.29-1+0~20200320.39+debian9~1.gbp513c2e"}]},"En-têtes de la requête (0 o)":{"headers":[{"name":"Accept","value":"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8"},{"name":"Accept-Encoding","value":"gzip, deflate"},{"name":"Accept-Language","value":"fr,en-US;q=0.7,en;q=0.3"},{"name":"Authorization","value":"Basic WHU2OXU3V2M6NyhoRDdDZUMjODl5TC1p"},{"name":"Connection","value":"keep-alive"},{"name":"Cookie","value":"fc_columnchooser=%7B%22vhash%22%3A%222d0b97fae34827186a4ad554914feece%22%2C%22adminListTableFCtypes%22%3A%222%2C3%2C4%2C5%2C6%2C7%2C8%2C9%2C10%22%2C%22adminListTableFCcategories%22%3A%222%2C3%2C4%2C5%2C6%2C7%2C8%2C9%2C10%2C11%22%2C%22adminListTableFCitems%22%3A%223%2C4%2C5%2C6%2C7%2C8%2C9%2C10%2C11%2C12%2C13%2C14%2C15%2C16%2C17%2C18%2C19%22%2C%22adminListTableFCfields%22%3A%222%2C3%2C4%2C5%2C6%2C7%2C8%2C9%2C10%22%2C%22adminListTableFCfileselement_default_25%22%3A%222%2C4%22%2C%22adminListTableFCsearch_advanced%22%3A%220%2C1%2C2%2C3%2C4%2C5%2C6%2C7%22%2C%22adminListTableFCsearch_basic%22%3A%220%2C1%2C2%22%2C%22adminListTableFCtags%22%3A%221%2C2%2C3%2C4%2C5%2C6%22%7D; fc_managers_conf=%7B%22vhash%22%3A%222d0b97fae34827186a4ad554914feece%22%7D; fc_screen_resolution=1680x960; fc_uid=047606ef89971d7f1420436044d87df7; fcfavs=%7B%7D; cookiesck=yes; d77a40890ae8276cfdcfe657d6e6806e=h9ut766ejb7lbk75kibr34p1mj; eb72b2ea1be9a31eca8642076503b43a=p2mrr22mpcdcf7l7psh5dmfjpf"},{"name":"Host","value":"airdf.ouvaton.org"},{"name":"Referer","value":" airdf.ouvaton.org/administrator/index.ph..."value":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:76.0) Gecko/20100101 Firefox/76.0"}]}}

2 - Filter on : /administrator/components/com_flexicontent/tasks/core.php?225bda64c3cbff5614ca841918448a4d=1
Not Found
The requested document was not found on this server.
Web Server at

{"En-têtes de la réponse (237 o)":{"headers":[{"name":"Accept-Ranges","value":"bytes"},{"name":"Content-Length","value":"1027"},{"name":"Content-Type","value":"text/html"},{"name":"Date","value":"Wed, 06 May 2020 15:34:23 GMT"},{"name":"ETag","value":"\"403-501d51bd29280\""},{"name":"Last-Modified","value":"Sat, 30 Aug 2014 09:08:10 GMT"},{"name":"Server","value":"Apache/2.4.25 (Debian)"}]},"En-têtes de la requête (1,466 Ko)":{"headers":[{"name":"Accept","value":"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8"},{"name":"Accept-Encoding","value":"gzip, deflate"},{"name":"Accept-Language","value":"fr,en-US;q=0.7,en;q=0.3"},{"name":"Authorization","value":"Basic WHU2OXU3V2M6NyhoRDdDZUMjODl5TC1p"},{"name":"Cache-Control","value":"max-age=0"},{"name":"Connection","value":"keep-alive"},{"name":"Cookie","value":"fc_columnchooser=%7B%22vhash%22%3A%222d0b97fae34827186a4ad554914feece%22%2C%22adminListTableFCtypes%22%3A%222%2C3%2C4%2C5%2C6%2C7%2C8%2C9%2C10%22%2C%22adminListTableFCcategories%22%3A%222%2C3%2C4%2C5%2C6%2C7%2C8%2C9%2C10%2C11%22%2C%22adminListTableFCitems%22%3A%223%2C4%2C5%2C6%2C7%2C8%2C9%2C10%2C11%2C12%2C13%2C14%2C15%2C16%2C17%2C18%2C19%22%2C%22adminListTableFCfields%22%3A%222%2C3%2C4%2C5%2C6%2C7%2C8%2C9%2C10%22%2C%22adminListTableFCfileselement_default_25%22%3A%222%2C4%22%2C%22adminListTableFCsearch_advanced%22%3A%220%2C1%2C2%2C3%2C4%2C5%2C6%2C7%22%2C%22adminListTableFCsearch_basic%22%3A%220%2C1%2C2%22%2C%22adminListTableFCtags%22%3A%221%2C2%2C3%2C4%2C5%2C6%22%7D; fc_managers_conf=%7B%22vhash%22%3A%222d0b97fae34827186a4ad554914feece%22%7D; fc_screen_resolution=1680x960; fc_uid=047606ef89971d7f1420436044d87df7; fcfavs=%7B%7D; cookiesck=yes; d77a40890ae8276cfdcfe657d6e6806e=h9ut766ejb7lbk75kibr34p1mj; eb72b2ea1be9a31eca8642076503b43a=p2mrr22mpcdcf7l7psh5dmfjpf"},{"name":"Host","value":"airdf.ouvaton.org"},{"name":"Upgrade-Insecure-Requests","value":"1"},{"name":"User-Agent","value":"Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:76.0) Gecko/20100101 Firefox/76.0"}]}}

Hello

the url is correct but it seems the file is missing !

please re-apply v3.3.4 package !!
(re-applying the same package does not create any issue !)

In Joomla installer
use install via URL
github.com/FLEXIcontent/flexicontent-cck/archive/v3.3.4.zip

It's running.
Think you for the lesson
JPS