Welcome to Microsoft Q&A forum.
As I understand, you are getting collation error in migration of Single to Flexible Server.
We are sorry about the inconvenience.
There is a similar Q&A post here:https://learn.microsoft.com/en-us/answers/questions/1336701/collation-encoding-error-when-trying-to-migrate-fr?cid=kerryherger
where the solution workaround is mentioned and recommended to raise support case for further assistance on the issue.
Please take a look and let us know if you need any help.