Unable to update connection credentials. Logging into Sql server failed.

Hi subhra1131101,
As mentioned by mopowerbi If feasible, consider creating a new connection and reconfiguring the existing reports to utilize it.
If it is not feasible, Please consider reaching out to Microsoft Support. You can provide them with all the troubleshooting steps you've already taken, which will help them understand the issue better and provide a resolution. They might be able to identify something specific about your admin account setup or provide a solution that isn't immediately obvious.
Below is the link to create Microsoft Support ticket:
How to create a Fabric and Power BI Support ticket - Power BI | Microsoft Learn
Thank you