、。
If you fail, there's no do-over, so don't fail.
Literal
Failed [if-たら] redo [subject-が] cannot-do [reason-から], fail-[prohibition-するな].
やり直し 'redo, do-over' is 遣り直す nominalized. するな is the plain prohibition form — blunt and direct. The logic is circular and somewhat darkly humorous: 'you can't redo it if you fail, therefore don't fail.' The pressure of a high-stakes situation compressed into two clauses.