15 “After you have purified the Levites and presented them as a wave offering, they are to come to do their work at the tent of meeting.

Read Numbers 8:15 Using Other Translations

And after that shall the Levites go in to do the service of the tabernacle of the congregation: and thou shalt cleanse them, and offer them for an offering.
And after that the Levites shall go in to serve at the tent of meeting, when you have cleansed them and offered them as a wave offering.
After this, they may go into the Tabernacle to do their work, because you have purified them and presented them as a special offering.