Can't perform action error message

I have group of moms that they can ask their friends to join their pod but these users aren’t yet on the app. So when the new mom signs up and I need to match them and update the connecting moms together. However, I am running into an issue with an error “Action can’t be performed”. On the screenshot, I am adding the current user to the users field on the connecting mom that I find. I iterate through the connecting mom collection and when I find the right phone number then I want to add that mom to the connected mom so I know all the users/moms.

I am not sure why I am getting this error. This field is a many-to-many to the user collection.

Any help would be greatly appreciated it.