How do I add a chamfer to the inside of the frame of this cabinet door?

There are a lot of ways to do that. Since you are modeling the door as a single object, you could have drawn a cross section of the door with the chamfer and a rectangle to use for Follow Me. Now that you have the dor you could draw a small triangle in the edge and use Follow Me to sweep it around.

Another option would be to use Offset to create parallel edges around the inside edges of the frame. Then use Move with Autofold to move the inner edges toward the panel.

Follow Me:

Offset and Move with Autofold:

That last one never occurred to me and I wasn’t sure if follow me would work on the inside…
Thank you!