Codes P0191 and P0138?

The P0191 code for your 2011 GMC Terrain is a diagnostic trouble code (DTC) for Fuel Rail Pressure Sensor Circuit Range/Performance.

The P0138 code is associated with "O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

So, you replaced the fuel rail sensor, correct? Here is the sensor in question to make sure we are on the same page. If this is the sensor you have replaced and you have the P0138 code it means your fuel pressure could be too high which means the fuel pump needs to be replaced. To be sure let's check the fuel pressure, this guide can help:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

You can rent a fuel pressure gauge for free or cheap at the auto parts store.

Running - 345-690 kPa (50-100 PSI)
Not Running - 300-400 kPa (43-58 PSI)

Check out the images (below). Please let us know what happens.