HI. I bought a .ws domain and I redirected it to one of my affiliate links. My question is: Can I do SEO on it?
Technically yes. You can build links to point to the domain, but it is not worth spending time on a domain that does not have its own website.
You should at least do enough to get you indexed in search engines. Or else it will not have any use at all. I have several similar domains that I put one page on and link it to the main website instead of a redirect.