Add Troubleshooting info on Invalid Key Format

Adds additional troubleshooting information for
generating ssh key pairs using fido and u2f hardware
security keys.

doc/ssh/index.md

@@ -479,3 +479,19 @@ ssh: Could not resolve hostname gitlab.example.com: nodename nor servname provid
 ```
 
 If you receive this error, restart your terminal and try the command again.
+
+### `Key enrollment failed: invalid format` error
+
+You may receive the following error when [generating an SSH key pair for a FIDO/U2F hardware security key](#generate-an-ssh-key-pair-for-a-fidou2f-hardware-security-key):
+
+```shell
+Key enrollment failed: invalid format
+```
+
+You can troubleshoot this by trying the following:
+
+- Run the `ssh-keygen` command using `sudo`.
+- Verify your IDO/U2F hardware security key supports
+  the key type provided.
+- Verify the version of OpenSSH is 8.2 or greater by
+  running `ssh -v`.