。
You have to mind your manners.
Literal
Manners [in-に] be-careful [must].
礼儀 ('manners, courtesy') stands alone here, paired with the standard ~なければならない obligation form. The fuller 礼儀作法 (the compound combining 礼儀 with 作法 'etiquette/method') would deliver the same maxim with a slightly more comprehensive feel; choice between the two is mostly stylistic.