I tried to use the acf_add_options_sub_page to add an options page to an existing group of admin menu items that are grouped under one parent. It added the page however I can’t configure any fields for it. The selection says “No Options page exist”
So the question is since I assume I need a main level options page first, can I nest a main level options page under a parent admin item?
You should be able to add options pages and they should appear to select them. When are you adding the options pages? On what hook? There can be an issue where defining the options page too late will cause it to not appear as a selection to assign a field group to it.