Resetting Time Capsule as External BackUp and receiving error

Previously used Time Capsule for several years as wired backup for my Mac. Recently went to Mesh system and now wish to use my time capsule as wireless external backup. As I go thru the steps in disc utility to set up, I do select other options and request that it join my existing network. As I continue, my existing network is displayed and the setup gets to the point of "joining network" and then I get the below error. I"ve tried this several times but continue to get the error. I would appreciate any help to complete the setup. Thanks.