I'm getting the error "Select two edge loops" when trying to fill the inside of the barrel.
Grid fill still works on circles in 2.8, but only if there's just the right number of vertices. Try double checking that there are an even number.
I couldn't get it right, so I filled in an edge between top and bottom, subdivided and worked from there. Took a bit longer, but making progress nonetheless.
Great, that works too!
