Why is the "Insert" button in my full/mini editor greyed out?

The Insert button is disabled when the cursor is positioned in Google Docs document in a location that does not allow us to insert the math image. Examples of such locations are footnotes or Google Equations. Position cursor in a different location in the document - when the position is supported, the Insert button will change colour to blue.

NOTE: If you try pressing “Insert” button when its colour is grey, there will be a popup message showing the details of why insert is invalid in the current position of the Google Docs document.