How come I can't add this specific song to my favorites playlist?

Hi ​@peachesandpenumbras  !

Maybe You cannot add a song to a Deezer playlist primarily due to regional licensing restrictions, the song being removed from the catalog, playlist size limits , or minor app bugs.

Here are the specific reasons and solutions:

Regional Restrictions: The song may not be available in your current country, which often happens with content licenses .

Playlist Size Limits: Your playlist might have exceeded the maximum recommended limit, which can cause issues with adding new content.

Technical Glitches: The app might need a cache refresh or a log-out/log-in to fix bugs affecting playlist modification.

Delayed Sync: Sometimes added songs take a few minutes to appear on the web version of the app. 

Troubleshooting Steps:
Clear Cache: Go to Settings -> Apps -> Deezer -> Storage -> Clear cache.
Log Out/In: Re-authenticate to resolve session errors.
Check Song Availability: Try adding a different song to see if the issue persists.
Create a New Playlist: Attempt to add the song to a new, smaller playlist.

Update Deezer app

If this above do not solve your issue , contact Deezer support

https://support.deezer.com/hc/en-gb/requests/new