Hi Francisco,
Please ask your question in the Microsoft Credentials forum using link below. There are support moderators over there that will respond to your question in 1 business day and can help fix the error that is preventing you from registering:
https://trainingsupport.microsoft.com/en-us/newthread?threadtype=Questions&forum=mcp
Please click Accept Answer and upvote if above was helpful.
Thanks.
-TP