Hello Ignacio Dinchong
Thanks for your question
This happens if If your account is associated with another tenant (e.g., your company’s), the sandbox environment can’t grant access. Personal accounts often resolve this automatically
Can you please try the work around solution suggested by TP here?
https://learn.microsoft.com/en-us/answers/questions/2184138/learn-sandbox-sign-in-button-not-working
It resolve the issue in this scenario, so you can give it a shot since you have already done the basic troubleshooting
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola