Transmission is stuck in S mode gear

Couple things. First and foremost, if this is still covered under the powertrain warranty then you will want a dealer to inspect it and tell you if it is covered. Clearly you don't want to pay for anything or even have to mess with it if it is covered under warranty.

If this is not under warranty we need to start with checking for codes. I attached a guide here that will help with that.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Then we need to run through this testing depending on what codes we have. I attached the testing that we need to run through below if there are no codes.

Here is a guide that will help with wiring testing if you are not sure how to take these types of readings:

https://www.2carpros.com/articles/how-to-check-wiring

I suspect you will have some codes and I think they will be CAN network codes. That is the communication network that is used by the various modules to communicate. Here is a guide that talks more about this:

https://www.2carpros.com/articles/can-scan-controller-area-network-easy