InReach Mini

I am trying to register a used unit as above prior to activating. I have inputted an IMEI Code verbatim with 16 digits. Try to move on with the next button but the explore.garmin site blocks me saying the IMEI code must be at least 15 digits. 16 is more than 15!!!

I haven’t purchased a plan as I assume it needs to be activated first. It’s driving me mad. 

any help please.