Correct spelling, explanation: in this case the core word guide was combined with the suffix -ance (and we received the word guidance). This suffix gives the meaning of process or action. The vowel e in the word guide was omitted as it encountered an -a in the suffix. Guidance is the correct final form of this noun, not guidence as it may be mistaken.

Definition of guidance:
noun, an action of leadership; also advice
I wouldn’t make it through this course without your guidance.
If you need any guidance let me know.

Incorrect spelling, explanation: the correct word, guidance is a noun created from the verb guide by using the suffix -ance. We drop the e at the end when we use the suffix. That’s why it is incorrect to spell it as guidence (e dropped and there is an a from the suffix instead). Guidance is therefore the correct spelling.