How can the A'raf — which is a barrier between Paradise and Hell — exist when Paradise is in the seventh heaven and Hell is in the seventh earth?

There is no definitive text that specifies the location of Hellfire now. Scholars have differed regarding its location. Some believe it is in the seventh earth, while others believe it is in the heavens. Some have refrained from stating a definitive location due to the absence of a conclusive text. The most probable approach is to refrain from asserting its location due to the lack of a conclusive text and the differing narrations from the Salaf.

As for Al-A'raf, it is a place between Paradise and Hellfire, a high wall from which its inhabitants look upon the people of Paradise and Hellfire. Then Allah will admit them into Paradise. The people of Al-A'raf are those whose good deeds and evil deeds are equal. This wall will be erected on the Day of Judgment, when the earth and the heavens will be replaced.