i have a fresh install of joomla 3.9.18 and successfully installed your extension, using web install. When i enable multi-site, i get mysql error 1064 and i get corrupted all basic joomla installation and sites. Luckily it is a vm and i can revert to last snapshot.
Tried also manually with downloaded file (pkg_seblod_3.18.2.zip) from your website with the same result.
Cleared cache after install as shown in video tutorial
Actually, I installed SEBLOD as first and unique extension. When i enable Multisite from Global Config > Seblod 3.x > Multi-site tab, just clicking on Yes button and then Save & close i get:
An error has occurred.
1064 You have an error in
your SQL syntax; check the manual that corresponds to your MySQL server
version for the right syntax to use near 'groups, public_viewlevel,
viewlevels, configuration, options FROM #__cck_core_s' at line 1
Also, any webpage on joomla server report 404 error and what i can do is to revert to last snapshot. I actually did not investigate more.