If your target clients speak Spanish then it would be vital. In my country it would be an unused skill as Spanish speakers are very low in number.
But, if you do need to learn the language to do business you need to be extremely fluent. Broken Spanish wont sell products. Being able to share a joke or some pun that works in the language might.
First you need to work on your sales pitch. It isn’t clear what you are offering? Web design? Hosting? Site management? Social media promotion? Social media management?