If you use 'on sale' the subject should be the products: "Clothes (are) on sale today!" "All jeans (are) on sale today!" If the shop or the people running it are the subject you should use 'have'. "We are having a sale today" "The shop is having a sale today"

You may say, "we are having sale"

I would not say "Our store is on sale". If everything in the store was on sale you might say, "We are having a store-wide sale."