Asked to perform task I lack skills in I am a undergrad student of biology, I was a computer science major but flunked out. One of my professors teaches/is learning genomics and wanted me to help him with a project since he assumed I had coding experience/skills. I told him that I lacked such skills and would not be a great fit to help him. Even as he described the project and his goals, I found myself stressed due to all the things he expected that I could assist with / do, so I am not really self-deprecating. 

I am interested in genomics, but I was going to build my coding skill set slowly through following books and the like. My professor expects him and I  to tie up all of his research during the next spring, and I doubt I could learn how to work with big data and all the possible languages required in such a short amount of time(for me).

Should I try and "bite the bullet" and learn the task(s) he has at hand to finish the project or should I decline his offer on the basis of not being skillful enough to be of use?
Think for yourself. If you're very much interested in Genomics, then working with this Professor would be beneficial for you in the long run. But you're not at all interested in learning those programming languages, then it would be better if you decline his offer. Please keep in mind that you won't always get such an opportunity. So, think we'll and decide. All the best!