Galaxy J7 began showing a Sim card error while I was on a recent trip. I've tried pulling the card out, re-inserting it and rebooting the phone, without success. I'd very much appreciate your advice on how to remedy the problem. Solutions I've viewed online apply to some other Samsung Galaxy phones, but don't work on the J7. Thanks!
Try another card in the phone and try that card in another device. If other cards work in the phone, then the phone's port isn't the issue. If the card works in other devices, then the card is fine. You would then need to see what could be on the phone that is causing the issue. Perhaps bad app, malware, virus, etc. Should the card not be readable on other devices either, then there is a problem with the card and the data on it will be lost.
If you can read it on other devices, I would suggest removing the data from the card, or at least copying it from the card to another device, a computer, etc., and then format the card and see if the phone will accept it after that. In the end though, you are probably in need of a new card soon.