Why was Moses unable to enter the tent of meeting?
Moses could not enter because the cloud covered the tent of congregation and the glory of the Lord filled the tabernacle. That cloud was the very presence of God, called the Shekinah glory, resting over the Holy of Holies. Once everything was set and God's presence abode there, even Moses was barred.
Les explains that after Moses finished setting up the tabernacle complex, it was ready for worship. Then a cloud covered the tent of the congregation, and the glory of the Lord, or Jehovah, filled the tabernacle. Because that cloud was the very presence of God—the Shekinah glory—Moses was not able to enter. Les notes this is the same cloud that first appeared when Israel came out of Goshen and camped at the Red Sea, protecting the Jews while becoming darkness to the Egyptians, and at night becoming the pillar of fire. Now it came and rested over the Holy of Holies, where the Ark of the Covenant was, filling that back room of the tabernacle. The tent was to be left for the priest, and even Moses, great man in God's service, could not go in now.
And Moses was not able to enter into the tent of the congregation, because the cloud abode thereon, and the glory of the LORD filled the tabernacle.