Test and see. Could be anywhere between the sensor in the transfer case to the actuator.

Perhaps start with making sure the transfer case is shifting into 4wd?

 

Pull out the front actuator shift into four-wheel drive and make sure that it is in fact working

 

2017 gmc 2500hd front actuator not working for 4x4. Transfer is locking in. Replaced switch in transfer and actuator up front axle. Have power and ground at actuator. I'm losing signal to actuator. Any ideas? Transfer is locking in. No 4x4 lite either.

 

So, you've removed the actuator from the front axle, plugged it in, and shifted between 2wd and 4wd, and it's not extending, either with the new actuator or the original one?

 

If your answers to Dave are in the affirmative, that would point to the switch on the dash. You could eliminate all this guesswork with a scan tool to pull the codes for it. They'll be there.