What Donald Trump said:
What's going on with FOX? What's going on there? They're putting more Democrats on than you have Republicans. Something strange is going on at FOX! Something very strange. Did you see this guy last night? I did want to watch, you've always got to watch the competition if you call it that. And he was knocking the hell out of FOX and FOX is putting him on. Somebody is going to have to have to explain the whole FOX deal to me.
In that quote, "this guy" refers to Pete Buttigieg, who is a Democratic candidate for president in 2020.
Looking just at presidential town halls on Fox, obviously there are going to be more Democrats than Republicans because twenty-three major candidates are running for the Democratic nomination and only two for the Republican (Trump and William Weld). Major defined by 538.com standards, primarily either previous government experience or meeting the minimum requirements to appear in the first Democratic presidential debate (Marianne Williamson and Andrew Yang). And the one participant who is running as an independent is a life long Democrat (Howard Schultz).
Of course, there is no requirement that town halls only cover presidential candidates. For example, Paul Ryan appeared on a town hall in 2017. Another 2017 town hall didn't have an attending candidate. Another had an administration official.
Perhaps Trump is hinting that he believes that Fox News should engage more with his administration in town halls. Or with congressional Republicans.