The short answer
First check that your sign-in matches the invited email address and that you are using the latest valid link. Check spam and company quarantine, the member list, project sharing, and your role. CALC checks that the email address is correctly formatted and reports an error if the invitation email cannot be sent. It does not check whether the address exists or whether the email reaches the inbox, so an invitation to a mistyped address stays pending. If the issue persists, give support the team name, affected account address, time, error message, and project name where relevant. Do not send passwords or complete invitation links.