Your SFTP server does not support calculation of checksums.
See https://winscp.net/eng/docs/library_session_calculatefilechecksum#remarks

First of all, very thanks for this amazing software.
My problem is - when I am trying to calculate checksum for a remote file, it shows operation not supported.
Is this happening because md5 checksum generator is not installed on my linux server or winscp is not able to calculate checksum of remote file.
I am trying to calculate checksum of remote file which is present on my linux sftp server.
Is there any other way we can calculate checksum for the remote file before download file locally?

I am using .Net library and the code is
var checksum = session.CalculateFileChecksum("md5", Properties.Settings.Default.ftpServerModDir);
The first string is the algorithm for calculating the checksum, the second is the remote path.

If I run this code, I get the following exception.
Operation not supported.

Thanks in advance!