Moses was unable to enter the Tent of Meeting because the cloud had settled on it, and the glory of the LORD filled the tabernacle.
As when, in the creation, God had finished this earth, which he designed for man's habitation, he made man, and put him in possession of it, so when Moses had finished the tabernacle, which was designed for God's dwelling-place among men, God came and took possession of it.
Commenting on Exodus 40:34-38
Moses was not able to enter into the tent of the congregation--How does this circumstance show the incapacity of man, in his present state, to look upon the unveiled perfections of the Godhead! Moses could not endure the unclouded effulgence, nor the sublimest of the prophets (Isa 6:5).
Exo 40:34-38 When the sanctuary, that had been built for the Lord for a dwelling in Israel, had been set up with all its apparatus, “the cloud covering the tabernacle, and the glory of Jehovah filled the dwelling,” so that Moses was unable to enter.
Commenting on Exodus 40:34-38