Limitations to merging faces?

I know this question has been posed before, but I haven’t seen anything like a concrete answer.

What are the limitations to merging faces?

Here’s the simplest manifestation:
Take a standard cube, then insert faces on any face of the cube. Now, the original cube face has 5 faces, one at the center, 4 along the edges. Try merging the 4 edge-faces, leaving the center face alone. It is disallowed. (“Could not create merged face”). Why couldn’t blender merge all faces?

Essentially, I need a face inside a face in my build - something like so (sorry not so great representation, but it does the job, I think) How can I achieve this?

| __ |
| || |

I figure I am somewhere in between beginner and intermediate in terms of Blender experience :slight_smile:

Much thanks in advance

The centre face needs to be connected by at least two edges