Were you even using any texting app on your tablet before you started using Go SMS Pro? I'm almost getting the impression you just installed the app onto it but without a previously established cellular account set up on your tablet, that's the problem. Any texting app requires an active phone number to work off of.
However, if you were using a different texting app before and just want to switch to Go SMS Pro as the new default texting app, try going into your Settings menu, open the Apps menu, find the texting app you were using and open it, then clear its default open settings. Now open the Go SMS Pro app and if you're asked if to allow it be used as the default texting app tap on yes.
It really would help if you would state what tablet model you have and which version of Android it's running. There are a lot of variables involved.