You say you've tried everything, but the example you provide is giving a very long path to the file.
The first step is verifying that you can access the file on the source server.
cat /home/server/EasyRSA-v3.0.6/pki/reqs/client1.req > /dev/null
If that fails, you can't transfer it as the user you're logged in right now. If you can change your user to the owner of the file or to root, you will probably have better luck.
The second step is to make sure you can write to the target location
ssh email@example.com ': > /tmp/client1.req; ls -l /tmp/client1.req'
If you can't write to that location as the target user, you won't be able to copy the file there. If the redirect to the file failed but
ls still listed a file, you may need to delete that file before you can copy this new file to that spot. Or maybe you need to give it a new name why you copy it.
Assuming that both of the tests above succeeded, but the scp is still failing, then take a look at the output from
ssh -v /home/server/EasyRSA-v3.0.6/pki/reqs/client1.req firstname.lastname@example.org:/tmp
If that doesn't show an obvious problem, it may help if you can add the output from that, starting right after it indicated the password was accepted (that is, everything after
debug1: Authentication succeeded (password).) to your question as additional information.