So, you want to rotate buildings in Age of Empires 4? To be fair, quite a few base-building city-building games, rotating buildings is somewhat essential for optimal planning and placement. There are many reasons for both why a player would want to rotate a building, and for why a game would let the building be rotated.
In Age of Empires IV, however, it is currently not possible to rotate a building. You can rotate the camera perspective, so you will be able to see all four sides of any given structure and then some, but in of when you place it down on the ground, there's nothing you can do but accept the pre-set rotation.
At the moment, there's no real reason why this would be an issue. Barring legitimate imable terrain, the ground around a building will mould itself to the structure being placed. A grid system also prevents buildings from overlapping, and when it comes to production buildings like the barracks or stable, units don't appear from any fixed point. If you block up the 'entrance', units will simply spawn from another side instead.
I guess you could try to completely surround a production building to interfere with spawning, but then that's A: something rotation won't be able to help with anyway, and B: incredibly sus.
That's all there is to say on the subject. If you're looking for other advice, we have a guide on Abbasid Dynasty.