What is the correct transformation to express that entry to the building was not permitted?

The choice of "We were not allowed to enter the building" effectively conveys the idea that entry to the building was not permitted. It succinctly captures the essence of the restriction in a straightforward and common phrasing.

This transformation aligns well with the original statement about permission. It emphasizes personal experience ("we were") and uses the phrase "not allowed to," which directly communicates the prohibition without introducing any ambiguity.

Although several other options might seem valid, they vary in tone, specificity, or structure. This particular choice maintains a clear and easily understood relationship to the original notion of restricted access.