Hi Eric Malcolm:
Make sure the account you are trying to signup has not been used previously. The simplest way is to use a different email address because the email and credit card has to be fresh and not used anytime before.
If you still face issues, please post your question in the dedicated forum below.
https://trainingsupport.microsoft.com/en-us/mcp/forum/
Please 'Upvote'(Thumbs-up) and 'Accept' as answer if the reply was helpful. This will be benefitting other community members who face the same issue.