This is the way Google is going with their app Integrity Protection. It also helps us minimize hacked copies. 

For Blackberry devices, this is the solution:

1) Open the Power Centre / Power Center app
2) Tap "Manage Automatic Apps"

3) Find "Google Play Store"

4) Move the slider to the right so it is green (active)

Launch the Textra app and the issue should be resolved.

The same concept of ensuring Google Play is active also applies to other devices, however the specific settings may differ.

Still not working? Here's an older version of Textra SMS where Google have not applied the Integrity Protection checks.

1) Uninstall your Textra SMS.
2) Click to install http://adhoc.textra.me/Textra-45961.apk

3) Turn off Auto-updates in Google Play for Textra SMS.