The adverb 大不了 has the following two meanings:
1) "at the worst".
In this meaning, it emphasizes that if something goes wrong, a solution can be found.
In a sentence, 大不了 is placed before a predicate, which is expressed by either a verb or a verb phrase.
A verb phrase is a combination of a verb and an auxiliary word. The auxiliary word can be either an auxiliary verb or an object.
It's okay to get on the wrong bus. At worst you will come back when the bus turns around
zuò cuò chē méiguānxi, dàbùliǎo jiùshì zuò huíqù
It doesn't matter if you fail. You can just start all over again
shībài le bú yào jǐn, dàbuliǎo cóngtóu zài lái
2) "serious", "severe".
In this meaning, 大不了 is used in phrases like "it's no big deal", "it doesn't matter".
大不了 can be followed by the particle 的 .