I'm creating a character in a campaign where loads of content sharing is enabled, my wizard only has the ability to choose "Evocation" as the arcane tradition.

What do?

Under assumption you did as Arenlor suggests (because if you didn't then you're not creating a character in the campaign):

You can check that somebody has actually purchased the books that contain the other subclasses, such as PHB.

You can ask the players to check their entitlements are in sync by going here: https://www.dndbeyond.com/account/marketplace

You can ask the person with the Master Tier sub that had enabled content sharing to check their subscription hasn't expired. 

You can ask the person with the Master Tier sub that had enabled content sharing to disable then re-enable the content sharing.

If all of the above fails, raise a Support Ticket using the Contact Us button at the bottom of the page. Allow a few days, or a week, for a response.

While it will not solve the problem for all sharing, you can buy either the sourcebook containing the subclass you want to use or just purchase that subclass specifically. At least this way you get to use it while waiting for the Customer Support. 

Yes, I did use the link provided by the DM for the campaign.

Disabling and Re-enabling content sharing for the campaign worked perfectly!  Thank you so much!