Convert a text frame to an object frame not working

[karenb2]

I have multiple documents previously worked on in InDesign 2021. The files have master pages that have many image boxes on multiple pages. In the past I was able to import a photo into these boxes without any issue, but I no longer can since the boxes have suddenly been convert to what seems like text boxes. When I try to reassign the content to a graphic under object in the menu, the selection of "object", "text", "unassigned" are grayed out. How can I fix this without recreating all the boxes on my master pages. 

[SJRiegel]

The only time I have seen that option grayed out is when there was already text content in the frame. It could be just a space or a return character.