Both forms shown below are correct.

Correct spelling


Correct spelling, explanation: canceled is the form popularised by Noah Webster in his 1898 dictionary, as the correct American form of this verb. Webster transformed many British words into their simpler American spellings. This is why canceled is also correct and mostly used in the US.

Correct spelling


Correct spelling, explanation: cancelled is the form accepted in British English. L is doubled, because the word ends with the consonant-vowel-consonant combination (here: cancel ends with c-e-l), which means, if we add a suffix to such word, the last consonant is doubled. Therefore in the UK and most of the English-speaking world cancelled is the correct form.

Definition of cancelled/canceled:
verb, the past form of verb cancel, to call off, to annul something
The event was cancelled, since not enough tickets had been sold.
I canceled my plans yesterday because I was exhausted.