Slitherine

I discovered that during my conquest of Averine with Cloudfels that only Eagles and Harpies are allowed to enter the plains province of Elo Rance.
When starting a new game temporarily with Ravengard I was able to move the entire army into the province, so I wonder if this is merely an annoying glitch or Nation related.

Has anyone else have a similar problem before?

Your screenshot is showing that Elo Rance has a Cloud City spell active on it, which allows only fliers to attack it.

I dont have much experience with the magic system yet so I will keep an eye on this in the future. Thank you.

You can view any spell effects on a province by pinning the province card (clicking on a province), and then mousing over the spell icons on the terriain portrait of the province card. That icon will tooltip what spell is currently active, and detail the spell's effects.