、。
If you make a scary face doing a job like this, your customers won't come, you know.
Literal
This-kind-of job [with-で] scary face [object-を] do-[if-たら], customers [topic-は] not-come [conjectural-でしょう].
Workplace-advice delivered in keigo. いらっしゃる is the honorific verb used as 尊敬語 for 'come,' 'go,' and 'be' — here replacing 来る ('come'). いらっしゃらない is the polite negative: '[the honored customers] will not come.' Using keigo for the customers is the standard service-industry register, where customers are 'お客様' and their actions are always honored. 怖い顔 ('scary face') is a set phrase for a stern or frightening expression — the opposite of 笑顔 ('smile'). ~たら~でしょう ('if X, [then] probably Y') is a conditional with a conjectural consequence.