"Tap to Pay is still starting up"

The phone's reader starts in the background when the app opens, and needs the internet to do it. Press Tap to Pay on iPhone while that is still happening and a line above the menu says the reader is starting, or that Tap to Pay is still starting up. Wait a moment and press again. The button itself never changes.

If it keeps saying so, the phone is not getting online. Toggle wifi, or move to where the signal is, and the reader reconnects. Each tap needs a connection at the moment of the tap, and the reader disconnects whenever the phone loses signal or the app goes to the background, so the first sale after either takes a few seconds longer.

If Apple's Terms and Conditions appear instead, Tap to Pay was never finished on this phone. Accept them and the till opens.