The Team Invitation page shows why, under Invitation Error or, if you're signed in, in a pop-up:
"This invitation was sent to a different email address": you're signed in with another account, such as Google on a different address. Sign out, open the link again and choose Sign In to Accept or Create Account with the invited address.
"This invitation has expired or is no longer valid": invitations last 72 hours, and a link stops working once it's accepted or cancelled, or if its team was removed. Ask a team admin to invite you again.
"This team is no longer active": the team was removed.
For admins: "This email is already invited or a member" means that address has joined or has an unexpired invitation. To resend, first click Cancel next to it under Pending invitations. Expired invitations don't block a new one.
Details: How do I invite someone to my team? and Managing Teams.