When I create the cylinder, it has 8 segments, let's call the panels or 8 vertical edges. I then multiply that by 12, because I want each panel divided 12 times vertically, so I have enough geometry (edges or polygons) to work with.
The warning is just a warning. You can still click Ok, and that will create a cylinder with 8*12 =96 vertical divisions.

Then I want to apply internal circular symmetry and select the 1st face:

The numbered faces suggest the number of panels. I want 8 panels with 12 vertical divisions, so I click on the face labeled 8.
However, we have 96 vertical edges, and 96 can also be divided by 6. Then you'll end up with 6 panels with 16 vertical divisions. Or it can be divided by 16. Then, you end up with 16 panels with 6 vertical divisions.
I know I linked to the documentation in a previous post, but frankly, the documentation for this feature is incomplete and doesn't explain this well.