That a priest shall not enter the Sanctuary with dishevelled hair*So Maimonides explains it in his Sefer-ha-Mitzvoth, Prohibition 163

Leviticus 10:6

KJV

And Moses said unto Aaron, and unto Eleazar and unto Ithamar, his sons, Uncover not your heads, neither rend your clothes; lest ye die, and lest wrath come upon all the people: but let your brethren, the whole house of Israel, bewail the burning which the LORD hath kindled.

ASV

And Moses said unto Aaron, and unto Eleazar and unto Ithamar, his sons, Let not the hair of your heads go loose, neither rend your clothes; that ye die not, and that he be not wroth with all the congregation: but let your brethren, the whole house of Israel, bewail the burning which Jehovah hath kindled.

WEB

Moses said to Aaron, and to Eleazar and to Ithamar, his sons, “Don’t let the hair of your heads go loose, and don’t tear your clothes, so that you don’t die, and so that he will not be angry with all the congregation; but let your brothers, the whole house of Israel, bewail the burning which the LORD has kindled.

JPS

And Moses said unto Aaron, and unto Eleazar and unto Ithamar, his sons: ‘Let not the hair of your heads go loose, neither rend your clothes, that ye die not, and that He be not wroth with all the congregation; but let your brethren, the whole house of Israel, bewail the burning which the LORD hath kindled.