Hi Andres Esteban,
Welcome to Microsoft Q&A forum.
As I understand, you are performing Migration of SQL Server to Azure SQL Database using SSMA and getting error: windows logins are not supported in this version of sql server and is using SQL credentials.
Could you please try to add:
Trusted_Connection=False;Encrypt=True;
to your connection string and see if this helps.
Also try to set:
Integrated Security=False
this might also help.
Other helpful posts: https://stackoverflow.com/questions/64002224/ssms-windows-logins-are-not-supported-in-this-version-of-sql-server
Let us know after trying above if issue resolve.
Thanks