Invalid boundary region to join faces I have the following:



I'm trying to dissolve all of those edges so that face is unbroken, but when I select all of them and use Delete > Dissolve Edges, I get the 'Invalid boundary region to join faces' error.

If I dissolve them one at a time, it works, and I can get it down to two edges, like here:



But then those two get that error when I try to dissolve them both individually and at the same time.

How can I get rid of these edges and make this face smooth? This model is intended to be 3d printed eventually.
Having a hole in an NGon is not a good idea. faces have to be filled. An NGon is made of invisible triangles but the link between vertices has to exist! Leaving a few edges makes is possible without crossing the hole.

Your face is smooth (flat would be more exact), unless you want to subdivide it, what you'd better not do with an NGon. Having more edges is not a problem, it's a solution! it won't cause any problem printing it. (it still needs thickness)

NGons are just supposed to make things easier and clearer, some sort of shortcuts, but knowing what is topology and why you sometimes need more is crucial for doing things that looks good.