This will be of no real help, but here it is:
VERY, VERY WEIRD.
I ran into this issue today, for the first time on a new workbook I just created. I use the ribbon Page Layout>Page Setup >Sheet, but the "rows to repeat on top" field was grayed out.
I checked some older files. Same situation, even if it was a file that I have previously selected rows to repeat on top, I would see $1:$1 in the field, but the field was still grayed out.
Then, I decided to open a new workbook and before I did anything, I used the ribbon Page Layout>Page Setup >Sheet, The "rows to repeat on top" field was active so I selected the top row and saved it.
Now, no mater which file I open, the "rows to repeat on top" field is active.
As I said, I don't know how this will help anyone, but this was my experience.
VERY, VERY WEIRD.