Incorrect spelling, explanation: we don’t spell this word with suffix -able since this suffix is in most cases used with words that are left full form without the suffix (for example dependable without the suffix is a full word depend; visible without a suffix is just vis, thus not a word). Moreover, vision is a Latin-core word and with such words, we mostly use the suffix -ible. This is why visable is incorrect and visible is the right spelling.

Correct spelling, explanation: vision is the word of Latin origin and because of that, we used -ible suffix and got the word visible. Another reason is an unwritten rule saying that if we take the suffix out of the word, it is left unfinished (visible without -ible is just vis – not a core word). This is why visible is correct and visable is a mistake.

Definition of visible:
adjective, one that can be seen
Is everything on the board visible to people in the back row?
You have to put the opening hours information somewhere else on the website. It’s not well visible now.