If setting up sub and surrounds previously used with another Sonos soundbar you must first remove the sub so that it shows as “unused” and the surrounds must be removed and reverted so that they show as individual rooms ( not a stereo pair). After the above conditions are met go to the room with the Arc Ultra and choose add sub and add surrounds ( not necessarily in that order).

Edit: After reviewing your profile the Play1’s if being used as surrounds may or may not work as they are not officially supported by the Arc Ultra. 

Also (for others who may find this), make sure that the radio/wifi is not turned off on the Arc. It uses that to communicate with the surrounds and Sub.