I get a notification from an app called Hidden Menu that says "Invalid SIM Card" which is the same issue as before.
I know the SIM is active and works for this model of phone from the other carrier.
This error is either caused by:
1. a damaged sim card
2. in correct sim card
3. Phone isn't compatible with new carrier.
These are the only 3 reason why this error may accrue. Have you spoken with the new carrier?
The sim works correctly in another Moto G6 play that is already unlocked. It is the correct SIM card model. And the other carrier supports this model of phone.
The issue appears to be that the device is the device is not unlocking the UICC from Sprint. The other carrier has stated that as well.
Are there any logs I can provide that could be helpful?
Ok. Let's try resubmitting the unlock. It sounds like the phone didn't receive the unlock signal. I don't know why, because you've stated that all the steps on your end have been completed. I sent you a PM so I can gather a smidge of info.