Despite being a master tier subscriber dndbeyond is not allowing me to enable content sharing. I own all the content pn dndbeyond so its matter of not owning the content

(Figured out it was the 5 campaign only limit lol)

Have you hit your 5-campaign limit on content sharing?

I, too, am having this issue. I thought that the 5 campaign limitation may be the issue, so I disabled sharing on two of my 5 games and still do not have the option to enable it in my new campaign. 

Has your subscription lapsed?

No. It's active.

Double check that your Account Entitlements are in sync: (https://www.dndbeyond.com/account/marketplace).

Yes. Shows Master tier and I can see all my purchases. 

Update: The "enable content" button has showed up so I've shared the content I want to, but my players are still saying they can't see any additional content.

What content specifically? Some content requires enabling it for the character using the toggles in the editor's home section. For example if you purchased and are sharing Eberron stuff, they will need to enable Eberron content on their character, to start seeing it.

(And, just to cover all bases: the characters must be made in the campaign in which you have enabled sharing)

Resolved!

Thanks, all for trying to help. Magically the "enable content sharing" button appeared late last night. As far as I can tell, it was just a glitch. I didn't do anything special to get this to reappear. I did, though, have to have all my players sign out and back in and they were able to see the additional content.