Dataflow refresh failure when appending (pro capacity)

I ran into this same issue, and this solution worked perfectly. What's kind of weird is that I have plenty of Dataflows (with Pro License) in which I use the same technique of appending different queries, but making sure some the upstream queries are disabled to avoid the final query becoming a Computed Entity (which is not allowed on a Pro License). I ensure all queries within the Dataflow have the same Privacy Levels so I don't encounter any issues when appending them...and all works just fine.
In my experience, despite taking all the above mentioned precautions, we sometimes run into the issue the original poster presented because some queries you are trying to append might be queries you have built from scratch (think of Date Table), which you then try to append to another query sourced let's say from an Excel File which you've set a Privacy Level of "Organizational". Your Dim Date query built from scratch doesn't have a Privacy Level setting (it's literally impossible to set Privacy Level setting for queries that you build from scratch) and so Power BI doesn't know if this query built from scratch contains sensitive information or not, and so it throws you an error message as the original poster stated. And so, as the original poster, replied back with the solution to his problem, you must "

• Go to Privacy and check "Allow combining data from multiple sources" within the Dataflow. This is basically telling Power BI that it's okay to combine/append your 2 queries (the one of which built from scratch in which Power BI can't tell what level of privacy it should be assigned).