Thank you for the screenshots.
Your files have the .PFILE extension, for example alina3.xlsx.PFILE. This means they were protected using Microsoft Information Protection / Rights Management, rather than BitLocker or Windows EFS.
This also explains why Encrypt contents to secure data is unchecked and why the Details button is unavailable.
Please do not try to open the .PFILE files directly with Excel or Paint and do not rename or remove the .PFILE extension. Excel's import screen and Paint's error are expected because those programs cannot directly read the encrypted file.
Please make sure the Microsoft Purview Information Protection Viewer is installed > Right-click one of the .PFILE files > Select Open with > Information Protection Viewer > When prompted, sign in using the exact Microsoft account or email address that was used when the files were originally protected.
Since you mentioned that the Viewer does not allow you to sign in with your personal Microsoft account, could you please send me a screenshot of the exact sign-in error message you receive? Please hide your email or any other personal information if it appears.