I can’t upload Pages document onto a website

The site tells you explicitly what file types it expects, and .pages is not among them. Open the Pages document in Pages, and export as PDF, or DOCX, and providing you do not exceed the site's size limitations, try again.

Well, what filetype or extension is it, And what site is it so we might find out what filetypes it allows.

Finding a site that knew what a .pages file is would be rare if not impossible.