Can't complete the Unbiden crisis - unconquerable planet

To answer your question, no, there is no way of fixing this.

As quoted below from the official Stellaris Wiki (Crisis), normally once an Unbidden reaches a habitable planet, that planet becomes a barren world:

If any fleet reaches the orbit of a habitable planet the planet will be stripped of all life and turned into a gray barren world that can be terraformed.

Now because this scenario didn't happen in your current game, and I have personally never experienced this case nor ever heard anyone experiencing this, I am assuming that is unfortunately a bug. My guess is the planet in question should have become a barren world but that for some reason it is in an odd state where you interact with it.

You can read more information about the Unbidden here.

Additionally, since you are playing in Ironman mode, you can't revert back to older saves. Indeed, as quoted from the official Stellaris Wiki (Ironman), there is only a single encrypted save file.

Only a single save will be stored; it is thus impossible to load an earlier save if something goes wrong (unfortunately, this includes save game corruption).

The game saves automatically whenever almost any decision is taken (such as declaring war).

The game saves automatically whenever almost any decision is taken (such as declaring war).

This file is accessible, but even if you do access it it won't be of any use since the game keeps overwriting that save file, so the last state where your game didn't have the bug has probably been overwritten for a long time.

If you still wish to retrieve the save file if you believe you can find a way to edit it, check this post, or read more about the file location here. But I would just keep on playing if I were you and assume that the planet has been affected by the Unbidden as it should have been.