Not able to activate "Invalid Unit Number"

This is a fairly unusual error message. If you are sure you've the IMEI and auth code correctly, I'd suggest opening a support ticket on this one.

You can also try clearing the browser cache, then try again. 

Where do I type in the IMEI and code?

Thanks 

Before you can activate, you must create an account at explore.garmin.com, choose an inReach plan, pay for it. Then you can activate.

As part of the new account creation, you will enter the IMEI and auth code.

This is probably not required when using the Messenger app (since the unit must already paired with the app). I do not know for sure. If there is a way to do something without using an app, I do it that way. The Garmin app ecosystem is way too cluttered and confusing for my tastes.

Thank you so much