> Could Germany build this Ratte?

Maybe. There is some debate about whether or not the Ratte ever got as far as serious design stage or if it was just some engineer at Krupp who decided to draw a ship that would go on land for his own personal amusement. That said, the Ratte would have been a very difficult project to say the least.

Mounting a 28cm SK C/34-guns might look possible on paper but realistically, I'm not so sure it would have been possible. A battleship turret is a bit like an iceberg, you really only see a small part. The loading mechanism, storage and rotation machinery (keep in mind we're talking a turret that weighs 300 tons) are huge. In the original configuration aboard Gneisenau the turret was five decks deep. Every shell for a 28 cm gun weighs around 300 kg, heavy enough to require purpose built machinery to move and the same is true for the charges.

In theory, all of these things could be solved but it would require massive reengineering of the existing turret support systems. The drive train, wheels and tracks of a vehicle of that side would also come under massive stress from the weight alone but again, it might be possible. It would have been a massively costly project however and it wasn't as if Germany had an abundance of 28 cm naval guns just lying around so production would have been limited to say the least.

> Would this Ratte have any practical value for Germany in the battles of world war 2?

Perhaps in terms of propaganda, sure, but realistically, it would have been a wet dream for the Allies or Soviets if Germany had gone through with this project. Another fanciful design that would drain the already over-stretched resources of Germany and for what? The Ratte for all it's terrifying size and firepower would have been a sitting duck for aircraft. Easily destroyed by armor piercing bombs, highly limited in terms of strategic mobility and extremely costly. Keep in mind that even the Yamato and Musashi at ~70 times the size of the proposed Ratte were proven to be highly vulnerable to armor piercing bombs even though they had heavier armor, greater speed and far better AA-armaments.

Perhaps someone actually considered building it for real at some point but we know for certain that it wasn't long before cooler heads prevailed and the project was abandonned for the folly that it always was.