It sounds to me as if only accountants are allowed to enter figures in a particular field, or space. Eg, "For office use only," or something like that. It reminds me somewhat of the box at the end of your tax return which is to be used only by a third party who has prepared the form.
Of course, this would not be the natural way to put it, but perhaps this is a translation.
That possible meaning of "entrance" didn't even occur to me. I don't think any native speaker of English would use "entrance" to refer to entering data on a form.
"Entrance for accountants only" isn't much of a sentence. It struck me as a translation from the beginning, but the OP doesn't give us much to go on. Back around the turn of the century, the more elegant bars had ladies entrances.