"This device can’t use 1krBlocker as its SMS app." means Android won't let any app become the default SMS app on this device, so 1krBlocker can't check messages before they arrive.
Common reasons:
- A tablet or device without a phone SIM, which can't send or receive texts.
- A work or school phone, where the organisation decides which apps may handle messages, or 1krBlocker is installed in a work profile.
- A restricted profile or child account, where the parent or owner controls default apps.
What to try: check that a SIM is in and working (you can text from the phone's own app), and that 1krBlocker is installed in your personal profile. On a managed phone, ask your IT department.
Without being the SMS app, 1krBlocker can't protect messages, because Android only lets that app see them first.