I had this problem with a Roku 4 Ultra Media Player.
I then bought a new power adapter, and hey presto. Straight through the update and activation. Now working fully updated fine.
This answered my question. I helped my aunt replace an older Roku Streaming Stick with a new Roku Express 4K+.
The new device was in an update loop as described by the OP. The original device was connected to the USB port on the TV for power, but this helped clue me in that the new Roku wasn't getting sufficient power. We just pulled out the power supply from the new box.
They were planning on relocating the old Roku to a rent house, but lost the original power supply since they haven't used it in the last 3 years. They had an old iPhone power supply that works just fine as well.