Psalms 99:6 NIV
Moses and Aaron were among his priests, Samuel was among those who called on his name; they called on the Lord and he answered them.
According to the law priests were consecrated and ordained into priesthood through a series of ceremonies and Aaron and his sons seem to be the ones that underwent this consecration and ordination
Exodus 28:1 NIV
“Have Aaron your brother brought to you from among the Israelites, along with his sons Nadab and Abihu, Eleazar and Ithamar, so they may serve me as priests
Exodus 29:1-4 NIV
29 “This is what you are to do to consecrate them, so they may serve me as priests: Take a young bull and two rams without defect. 2 And from the finest wheat flour make round loaves without yeast, thick loaves without yeast and with olive oil mixed in, and thin loaves without yeast and brushed with olive oil. 3 Put them in a basket and present them along with the bull and the two rams. 4 Then bring Aaron and his sons to the entrance to the tent of meeting and wash them with water
Leviticus 8:1-4 NIV
The Lord said to Moses, 2 “Bring Aaron and his sons, their garments, the anointing oil, the bull for the sin offering,[a] the two rams and the basket containing bread made without yeast, 3 and gather the entire assembly at the entrance to the tent of meeting.” 4 Moses did as the Lord commanded him, and the assembly gathered at the entrance to the tent of meeting.
There is no record of Moses undergoing a similar consecration and ordination
Was Moses a priest?.