What happens when the ET+ App is unable to recognize the REST API?
In the vast majority of cases, this is due to shared hosting environmental limitations and/or server settings. The likely cause of the error at this point would be that the host is not exposing the ticket check-in endpoint on their WP install properly.
The failure is when the response from the server is not JSON.
One user noted a fix was due to the following:
"It was resolved by increasing SUHOSIN settings on the account on the server."
Users can contact their hosting provider, informing them of the issue they are running into.
See also: Using QR codes with Event Tickets Plus