Hashem wanted the Mishkan built so thereby He would dwell in the midst of His people (Shemot 25:8). Now, after Moshe assembles the Mishkan, this goal is achieved:
And the cloud covered the Tent of Meeting, and the glory of Hashem filled the Mishkan. And Moshe was not able (YACHOL) to enter the Tent of Meeting, because the cloud rested upon it while the glory of Hashem filled the Mishkan. And whenever the cloud was raised up from upon the Mishkan, the Children of Israel would travel in all their journeys. But if the cloud did not rise up, then they did not travel, until the day that it rose up. For the cloud of Hashem was upon the Mishkan by day, and fire would be on it at night, before the eyes of all the house of Israel, throughout all their journeys (40:34-38).
Actually, for seven days, Moshe assembled and dismantled the Mishkan to teach the Leviim. On the eighth day, which is the first of Nisan, it is set up and not taken down, and that is when the glory of Hashem filled the Mishkan (see Ramban on 40:2, and the Midrashic sources he cites).

This is the end of the book of Shemot. The book of Vayikra begins:
And He called to Moshe, and Hashem spoke to him from the Tent of Meeting… (Vayikra 1:1).

A number of questions arise from this passage:
• Was Moshe either unable to enter the space occupied by the glory of Hashem, unable to see the entrance, or not permitted to enter the Mishkan?
• Is the beginning of Vayikra a continuation of the end of Shemot, or is it a separate incident?
• Why does the book of Shemot end with an excursus about the cloud (verses 36-38)?

There are a number of striking similarities between this passage and the description of Moshe on Mount Sinai:
And Moshe went up to the mountain, and the cloud covered the mountain. And the glory of Hashe rested upon Mount Sinai and the cloud covered it for six days. And He called to Moshe on the seventh day from the midst of the cloud. And the appearance of the glory of Hashem was like a consuming fire on the mountaintop, before the eyes of the Children of Israel. And Moshe entered into the midst of the cloud, and he went up to the mountain; and Moshe was upon the mountain for forty days and forty nights (24:15-18).
It is clear that the Mishkan has been constructed to duplicate and perpetuate the encounter between Hashem and man at Sinai.

It is also clear from the above that Moshe was capable of entering the cloud at Sinai. He was also capable of doing so in the Mishkan:
And when Moshe arrived at the Tent of Meeting to speak with Him, he heard the Voice speaking to him from upon the Ark-Cover that was upon the Ark of the Testimony from between the two cherubs; and He spoke to him (Bamidbar 7:89).
But, then what is meant by
And Moshe was not able (YACHOL) to enter into the Tent of Meeting?

One way of resolving this is found in Rashi, which is based on the Sifra (introduction, 8). Moshe was unable to enter the Mishkan only when the cloud rested upon it while the glory of Hashem filled the Mishkan. He would wait outside the Tent of Meeting until Hashem’s glory would withdraw. Then he would enter, and Hashem’s Voice would emanate from between the cherubs. Thus, the beginning of Vayikra, which depicts Hashem speaking while Moshe stands outside the Mishkan, occurred during the first seven days of installation.

Of course, expressions like the glory of Hashem filled the Mishkan and the glory of Hashem rested upon Mount Sinai should not be misconstrued to suggest that Hashem is corporeal. Rambam states this emphatically in The Guide of the Perplexed (I:8, 19, 25). “Hashem’s glory” refers rather to
“the created light that G-d causes to descend in a particular place in order to confer honor upon it in a miraculous way (ibid., 64).”

Nevertheless, this potential for misunderstanding might be why Ramban suggests another resolution:
“The reason for this was so that Moshe should not go in without permission, but instead G-d would call him and then he was to come into the midst of the cloud, just as He had done on Mount Sinai.”
According to this, YACHOL carries the meaning “permitted,” for which there are several precedents (Devarim 7:22; 12:17; 16:5; 17:15; 22:3).

In Ramban’s reading, the beginning of Vayikra occurs on the eighth day of training the kohanim and installing the Mishkan. What is more, the book of Shemot closes with Moshe standing outside the glory-filled, cloud-enveloped Mishkan, waiting for Hashem’s permission to enter, but
“Moshe did not enter the Mishkan, but Hashem called him from the Tent of Meeting and he stood at its door while He spoke to him.”

Until Hashem calls him, however, the Torah adds a parenthetical description of the cloud. Abravanel (Don Yitzchak Abravanel, 1437-1508) explains, with reference to the very beginning of the Torah:
“For just as Hashem, may He be blessed, is the universe’s soul and form, sustaining it and moving it forward, …so it was He, in His glory that abode on the Mishkan, Who sustained it and moved it forward.”

The universe was created by Hashem’s word alone. The Mishkan was brought into being by Hashem’s word, but this time was realized by the actions of man, the people of Israel.