This is the 1472nd most frequent Japanese word.
許さ
"許さ" (yurusa) means "to forgive" or "to allow."
許さなければ: This usage of '許さ' is in its verb form '許す' which means 'to forgive' or 'to allow'. The sentence implies the necessity to forgive someone's wrongdoing.
彼の行いを許さなければなりません。
We must forgive his actions.
許されない: Here, '許さ' is used as a negative potential form of '許す', indicating that something is not permissible or is unacceptable.
作業の中断は許されない。
Interruptions in the work are not allowed.
許された: This is the passive form of '許す', used to indicate that the proposal was approved (literally allowed) by someone of authority.