It seems the extension has a privilege issue. I tested the following on this thread: https://lepointdarret.com/public/d/50-calculer-le-nombre-de-jours-entre-deux-dates
The admins and moderators are able to see the button that would let them select an answer as best. That said, the OP is unable to do so himself. The forum permission settings clearly show that members should be able to select a best answer on their own discussions.
Below is my php flarum info
Flarum core 1.6.3
PHP version: 7.4.33
MySQL version: 10.3.37-MariaDB-log-cll-lve
Loaded extensions: Core, date, libxml, openssl, pcre, sqlite3, zlib, bz2, calendar, ctype, curl, hash, filter, ftp, gettext, gmp, SPL, iconv, pcntl, readline, Reflection, session, standard, shmop, SimpleXML, mbstring, tokenizer, xml, i360, bcmath, dba, dom, enchant, fileinfo, gd, imagick, imap, intl, json, ldap, exif, mysqlnd, mysqli, odbc, PDO, pdo_mysql, PDO_ODBC, pdo_pgsql, pdo_sqlite, pgsql, Phar, posix, pspell, snmp, soap, sockets, sysvmsg, sysvsem, sysvshm, tidy, timezonedb, xmlreader, xmlrpc, xmlwriter, xsl, zip, clos_ssa
+-------------------------------------+---------+--------+
| Flarum Extensions | | |
+-------------------------------------+---------+--------+
| ID | Version | Commit |
+-------------------------------------+---------+--------+
| flarum-flags | v1.6.1 | |
| flarum-approval | v1.6.1 | |
| flarum-subscriptions | v1.6.3 | |
| flarum-tags | v1.6.1 | |
| flarum-sticky | v1.6.1 | |
| fof-follow-tags | 1.1.7 | |
| flarum-suspend | v1.6.1 | |
| fof-impersonate | 1.1.0 | |
| flarum-lock | v1.6.1 | |
| flarum-likes | v1.6.1 | |
| flarum-markdown | v1.6.1 | |
| zerosonesfun-up | 1.0 | |
| zerosonesfun-expired-posts | 0.4 | |
| zerosonesfun-direct-links | 3.1 | |
| v17development-seo | v1.8.0 | |
| the-turk-stickiest | 3.0.1 | |
| the-turk-nodp | 1.0.1 | |
| sycho-move-posts | v0.1.7 | |
| sycho-advanced-extension-categories | v0.1.3 | |
| nearata-login-notification | 1.0.0 | |
| nearata-copy-code-to-clipboard | v2.1.0 | |
| isamuraii-op-dark-mode-tuning | 0.2 | |
| ianm-gravatar | 0.2.0 | |
| ianm-follow-users | 1.2.0 | |
| fof-user-bio | 1.1.1 | |
| fof-subscribed | 1.1.2 | |
| fof-stopforumspam | 1.2.3 | |
| fof-socialprofile | 1.1.4 | |
| fof-sitemap | 1.0.3 | |
| fof-share-social | 1.1.2 | |
| fof-secure-https | 1.1.0 | |
| fof-polls | 1.3.0 | |
| fof-nightmode | 1.5.1 | |
| fof-moderator-notes | 1.1.0 | |
| fof-masquerade | 2.1.3 | |
| fof-linguist | 1.1.0 | |
| fof-ignore-users | 1.1.0 | |
| fof-doorman | 1.1.1 | |
| fof-byobu | 1.1.8 | |
| fof-best-answer | 1.2.4 | |
| flarumtr-mobile-search | v1.2 | |
| flarum-statistics | v1.6.1 | |
| flarum-mentions | v1.6.3 | |
| flarum-lang-french | v4.6.0 | |
| flarum-lang-english | v1.6.0 | |
| flarum-emoji | v1.6.1 | |
| flarum-bbcode | v1.6.0 | |
| extiverse-mercury | 0.2.0 | |
| dem13n-topic-starter-label | 0.1.8 | |
| datitisev-backup | 1.0.2 | |
| clarkwinkelmann-status | 1.0.0 | |
| clarkwinkelmann-mailing | 1.0.0 | |
| clarkwinkelmann-lock-likes | 1.1.1 | |
| clarkwinkelmann-group-invitation | 1.0.2 | |
| clarkwinkelmann-emojionearea | 1.0.0 | |
| blomstra-fontawesome | 0.1.5 | |
| askvortsov-rich-text | v2.1.7 | |
| askvortsov-moderator-warnings | v0.6.1 | |
| askvortsov-markdown-tables | v1.2.1 | |
| askvortsov-discussion-templates | v0.8.3 | |
| askvortsov-checklist | v1.3.1 | |
| antoinefr-online | v1.0.1 | |
| acpl-mobile-tab | 1.1.1 | |
+-------------------------------------+---------+--------+
Base URL: https://lepointdarret.com/public
Installation path: /home/accolade/lepointdarret.com
Queue driver: sync
Session driver: file
Mail driver: mail
Debug mode: off