SessionRemoteException: Operation not supported.

I was using WINSCP.net library and when I tried to calculate the checksum value for a file at the sftp server, it can't and throws an exception as operation not supported. I have mentioned the algorithm as sha-1 and the code used is shown below.
There are no problem at sftp server as I can download the files and access them very well.

WINSCP version: 5.7

Have I done anything wrong over there.

Please help me on this and let me know how to fix this.

Thanks in advance.

WinSCP does not download the file to calculate the checksum locally. It asks the server to calculate it remotely. But your SFTP server does not support "check-file" SFTP extension.
See https://winscp.net/eng/docs/protocols

If you want to calculate the checksum locally, download the file and use SHA1.ComputeHash:
https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.sha1
https://learn.microsoft.com/en-us/dotnet/api/system.security.cryptography.hashalgorithm.computehash

See also https://winscp.net/eng/docs/library_example_verify_file_checksum