Readily is an adverb (adverbs modify adjectives) meaning “being at ready access”. It has no simple antonym (opposite like unreadily, which is not a word).

Available means that people are able to reach someone/something for some use or objective. It has that word able. Avail means “make use of.” Make-use-of-able, lol.

So it might appear to be redundant (repetitive), but it’s really just a common way to say that someone or something is very accessible, meaning if you need it/them, you probably won’t have to work very hard or go very far to get it/them.

"I was thinking of baking three or four loaves of rye bread, but I changed my mind because a good supply of rye flour wasn’t readily available at my favorite local market."

Please refer to:

https://www.google.com.vn/search?q="readily-available""luôn ...