I am in A Campaign with a DM that owns every sourcebook and Adventure and that has the Mastery Subscription. He has content sharing enabled and I have everything permitted in my Character Sheet. But if I want to choose the subclasses from the book he owns and if I want to view the sourcebooks and adventures he owns it doesnt work and it just redirects me to the Marketplace. Is it a bug or doesnt content sharing work anymore? If it is a bug how can I solve this issue? I already tried joining and leaving.

I am having the same issue

Check your entitlements (user menu->account->licenses)

Are you making the character in the campaign where the books are shared?

Quote from jl8e >>

Check your entitlements (user menu->account->licenses)

Are you making the character in the campaign where the books are shared?

 

 
In the unlikely event that this comment ends up not containing the solution to your issue, your DM might have selected to not share the compendium content for those books with you specifically to prevent you from reading ahead in their adventures, and your inability to select player options might be due to the information contained here: (https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/207687-read-this-if-you-cannot-see-all-your-subclasses).