- Truly is the only acceptable way to spell the adverbial form of the adjective true.
- Truely is not an alternative spelling; it’s a common mistake.
Some adjectives like nice, fine, and blue retain their final e when adding the adverb suffix -ly: nicely, finely, and bluely. With truly, this is not the case.
Is It Truly or Truely?
Although some monosyllabic adjectives ending with “e” retain it when they adopt the -ly suffix to become adverbs, true isn’t one of them. This might be the reason some people mistakenly think that truely is the way to spell the word. It’s not. Truly is the only accepted spelling:
Most of us make spelling mistakes from time to time. Some words seem to almost invite us to make mistakes. “Forty” and “fourty” are such a pair, “preferably” and “preferrably” are another, and there is always “foolproof” and “fullproof” to keep us on our toes.
from Grammarly Blog https://www.grammarly.com/blog/truly-truely/