John Gill Commentary 2 Chronicles 26:18

"and they withstood Uzziah the king, and said unto him, It pertaineth not unto thee, Uzziah, to burn incense unto Jehovah, but to the priests the sons of Aaron, that are consecrated to burn incense: go out of the sanctuary; for thou hast trespassed; neither shall it be for thine honor from Jehovah God." — 2 Chronicles 26:18 (ASV)

And they withstood Uzziah the king
They not only stood against him, but stood about him, surrounded him, so as to hinder him from approaching the altar of incense:

and said unto him, it appertaineth not unto thee, Uzziah, to burn
incense unto the Lord ;
it did not belong to his office as a king, it was no part of it

but to the priests the sons of Aaron, that are consecrated to burn
incense ;
and to them only; for even the Levites might not do it, only those of the tribe of Levi, that descended from Aaron, see (Numbers 16:35) ,

go out of the sanctuary, for thou hast trespassed ;
by going into that:

neither shall it be for thine honour from the Lord God ;
but to his hurt, and be a brand of infamy upon him; for more is designed than is expressed, and as the event showed.