I believe I have solved this PBS glitch.
I also ran into the problem of the PBS app just giving me a never-ending spinning circle when first installed on my Roku 3.
It appears the PBS app is very resource-hungry. So, I went through my Roku and deleted several other channel apps that I rarely (if ever) used.
That freed up more system resources.
Then, upon trying the PBS app again, everything worked fine. It loaded, went through the activation process, and is now working as it should.
I hope this information is helpful.