The username I am adding is the only administrator user which is my own account, however, it constantly coming up with the error message: “That administrator username was not found on this child site. Please verify that it is an existing administrator.”
I’ve added nearly a dozen other websites with this one being an exception.
What could be the cause or possible solution to this problem?
Make sure you use the correct uppercase/lowercase notation of the username. Mixing them up can cause this issue, because the check is (or was?) case sensitive.
If you would still be facing this issue after checking the username case as @josklever suggested, please open a private Help Desk ticket so we can collect some additional information & investigate further.
This was your first reply on the other question as well which isn’t the answer in this case either as it’s the only administrator and the username is perfect.
I will open a help desk ticket as suggested by @bojan