Document the process that enables a B2B partner invite
The B2B invite process is failing with "This invitation may not be used to signup a new user". What are the prerequisites for the invited partner organisation. What is being checked for the invited user?
Hans Worst commented
CcEmailAddress: Email address to copy emailed invitation. If the CcEmailAddress field is used, this invitation cannot be used for email-verified user or tenant creation.
And the e-mail address of the invitee shouldn't be already in your Azure AD or the import will fail with an error.