There has been a recently reported bug where a "not on trusted list" error message wrongly appears and profiles cannot be connected. For example see this G2G question. In one case the problem had gone away by the following day. Would people who see this like to try again soon, and let us know if you then succeed?
One important thing is to double-check you are entering exactly the correct WikiTree ID.
There is a plan to improve the situation, but I don't know if it's been implemented yet.
The frequency of occurrence of this bug seems to have escalated dramatically in the last few hours. See also here.
And here. Also at this link.