Explanation:
The error is in the last word.
Everyday is an adjective. In other words, it is used to add a characteristic to something. Generally, everyday is used to define something that you see every day. In other words, something typical, mundane, or common.
Every day, on the other hand is a phrase that means "each day" or "all days" depending on the context.
Answer:
The phrase should be written like this:
WE OFFER LOW PRICES EVERY DAY
(We offer low prices every day)
