Welcome to the world of radio
. What you have is a proper marine
station license/call sign. It's the right number of characters, etc.
If you are trying to set up RMS Express to use Winlink, however, it is looking for an amateur
(ham) radio call sign, which is a whole different beast.
You can (let debate begin) use the same SSB
radio in marine
and ham service
, but you will need separate licenses and call signs for the two separate services.
Edit - For SSB e-mail your marine
call sign would generally be used with Sailmail or one of the other similar services.