Hey, first of all - I am not XenForo staff. But, I have seen this before, so figured I will try help with advice.
As always, take a backup of DB, files etc. before doing anything more.
Now...
What I found previously is that your XenForo files may be mixed or stale from an older version. Article.php should not be present in a clean XF 2.3.11 install.
I would re-upload a clean, complete XenForo 2.3.11 file set, making sure existing files are overwritten and the 2.3.11 src/addons/XF/hashes.json is present.
Then from the XF root run:
php cmd.php xf:file-clean-up XF --dry-run
php cmd.php xf:file-clean-up XF
php cmd.php xf:upgrade
Because the upgrade steps already reached 2.3.11, xf:upgrade may ask to rebuild master data. Answer yes...