Correct spelling, explanation: according to the rule in case of words ending with -er sound AND with the stress not on that syllable, we don’t double the last consonant (r) when adding a suffix. In case of the stress on the last syllable, we do double the r, so in refer the last consonant will be doubled (just like in referral). Therefore referral is the correct spelling and referal is not.

Definition of referral:
noun, something or someone that is referred
They accepted our request for referral to a different department for our case.
Do you have a referral from your doctor?

Incorrect spelling, explanation: this form, referal, is incorrect as in the word refer the last consonant should be doubled when adding a suffix – hence referral. This happens because in refer the last syllable is stressed. We only do not double the last r in case of the stress on a different syllable (for example batter). Therefore referral is the only correct form.