Invalid SenderID

I’m trying to send SMS texts within the United States and no matter what I try, I keep getting this error: “Invalid Sender ID”
I use gateway 1s2u.com and I’ve tried talking with 1s2u’s support but still no luck. Things work just fine on their end but when I go to input things in my WP SMS plugin, it doesn’t work. I can’t seem to figure out what the Sender ID is. I’ve tried everything.

Any help would be greatly appreciated! Thanks!