You are most likely using an R80.x release, not R80 itself, which is only for Management and was deprecated long ago.
The remote server is rejecting connection because the two sides could not agree on what encryption algorithms to use.
This is expected behavior given the vintage of OpenSSH used on older releases.
See: https://support.checkpoint.com/results/sk/sk150674
Instead of initiating the scp from the device you want to upgrade, initiate it from an external device.
You can also use sftp: https://support.checkpoint.com/results/sk/sk82281
In either case, the user in question should have a bash shell instead of clish (step 7 of sk82281).