Hi, I'm having the same problem. Ive contacted support numerous times. I'm always told first that my device is already unlocked, them that it is in fact still locked and that it will be fixed soon. The problem persists. I can send you my support ticket number.
I had this problem with a samsung galaxy s8 this week. After going back and forth with Sprint and and the new carrier spending 3 hours on the phone, Sprint was very sure the phone was unlocked so they told me to go to a Sprint store for in-person assistance.
Before heading to the store, i randomly came upon a setting value under System Update named UICC Unlock. Clicking on this triggered an update and completed the unlocking process. Everything worked perfectly after that. No problem with the new SIM.
So, I am sharing this because I started the unlocking request on Sep. 19, 2019. Since then, i have been on the phone with Sprint support four times, including twice yesterday. Not once during any of the troubleshooting did anyone even mention this settings value. If it is not part of your trouble-shooting script, maybe it should be added.
The path is Settings --> System Update --> UICC Unlock.
The value of the setting will say "Your device is locked to Sprint SIMs only." NOTE: It says this even after verifying three times with Sprint tech support that the phone was unlocked.
Press on that setting and a pop up will open that contacts whatever database this update lives. After a few seconds, status will change to "unlocked" and the SIM invalid error resolves. The phone asks to be restarted to flash the new info from the new carrier.
I have a galaxy s7 and I went to the sprint store because it said my sim card was invalid. they got it to work, but after a couple of minutes, the sim card locks again. Help, please.