why is my galaxy s4 saying this sim card is from an unknown source

[Android Central Question]

when there is a valid sim card in the phone

 

[Dellsimkin]

If it's a large update they're giving you it could take an hour or two to complete in the SIM card wouldn't be registered to any phone at that moment check your theater service provider for that or it may be after it updates you might need to buy a prepaid card if you do thatyou need to call your service provider and check your account

 

[Rukbat]

Most likely because the SIM card has gone bad (your carrier or MVNO should replace it) or because the SIM port in the phone went bad (less likely, but possible).