Have to say I am having issues with an EU site version of WU lately (all of 2021 to date..) keep getting error messages saying to “complete all mandatory fields” on the final screen after submitting the transfer. All fields are complete, no specific field appears erroneous, doesn’t matter if using phone or computer, and their customer service never responds. If I try the same thing like 5 times in row, eventually it works most of the time.
Had something similar happen to me until I disabled Firefox's Enhanced Tracking Protection for the WU site. That and aggressive adblocking can usually break legitimate sites (still 100% worth it to keep it enabled and disable it on a per case basis).