Hi! Is the option to apply background effects missing for you in both the desktop and web apps? If it's desktop only- try clearing the cache (by deleting the files in these locations)
• %AppData%\Microsoft\teams\application cache\cache
• %AppData%\Microsoft\teams\blob_storage
• %AppData%\Microsoft\teams\databases
• %AppData%\Microsoft\teams\cache
• %AppData%\Microsoft\teams\gpucache
• %AppData%\Microsoft\teams\Indexeddb
• %AppData%\Microsoft\teams\Local Storage
• %AppData%\Microsoft\teams\tmp
If that doesn't work - try the trick from this thread:
https://answers.microsoft.com/en-us/msteams/forum/all/show-background-effects-option-has-disappeared...That forum is specifically for Teams issues, so you'll be able to get further help there :)