"If my company does not evade tax, it will go bankrupt."
Translation:Nếu công ty của tôi không trốn thuế, thì nó sẽ phá sản.
của is not necessary especially in colloquial speech.
but while your sentences are not wrong without thì, being a conjunction, thì connects the main and nếu clauses together and keeps the flow of your sentences.
ps. having already thì present, the comma is not a necessity.