Why can't I access/get into an Opportunity?

Opportunities can have multiple states for sign up. When an Opportunity is either Closed or Full, you will not be able to interact with the sign-up button to join the Opportunity. To get more information on the sign-up state of the Opportunity, you will want to contact the manager directly. 

Closed Opportunity with a disabled signup button.

Some Opportunities only allow signup for members of that Opportunity’s Organization. If you are not a member of that Organization, the signup process stops and a message opens prompting you to join the Organization first.

Message stating an Opportunity is limited to Organization members only.