Read Numbers 8:26 Using Other Translations
But shall minister with their brethren in the tabernacle of the congregation, to keep the charge, and shall do no service. Thus shalt thou do unto the Levites touching their charge.
They minister to their brothers in the tent of meeting by keeping guard, but they shall do no service. Thus shall you do to the Levites in assigning their duties."
After retirement they may assist their fellow Levites by serving as guards at the Tabernacle, but they may not officiate in the service. This is how you must assign duties to the Levites.”