Incorrect spelling, explanation: there is no need to double l in adverbial suffix -ly, if the word doesn’t end with l. This is why trully in an incorrect form. The correct one is truly – true plus suffix -ly (e is skipped probably due to pronunciation).

Correct spelling, explanation: truly is an adverb. We create adverbs by adding the suffix -ly to an adjective. In this case, the adjective is true; by adding the suffix e is skipped on. Usually, we don’t skip e if the suffix starts with a consonant, but modern spelling is truly, maybe because of the pronunciation. Trully is incorrect since there is not l in true, so we do not need to double it.

Definition of truly:
adverb, in a truthful matter
I truly love you!
Truly, this is the best play I’ve ever been to.

Incorrect spelling

Incorrect spelling

Incorrect spelling