Can I still publish (into a tangible book) a story if I upload it to fictionpress.com?

Yes, actually, but it may be more difficult. It's definitely not impossible, though. I know of a few people who have gotten publishing deals BECAUSE OF the success on FP, namely Myrika and sophiesix, as well as author-A whose first novel will be published in October. Mind you, as soon as they got a publisher they were asked to take the story down from FP, but it didn't really prevent them from getting published. It depends on the publisher, however.