You are not loosing clients because they are not your target group yet.
The clients that are asking for portfolio are looking for experienced webmaster. Sad but true, but most of them are not interested in hiring someone that hasn't done at least a couple of projects. They want to be sure that you really can do the things and secondly, they also want to see what's your style, to check if that passes to them.
What to do? You're a beginner, so you should consider taking significantly less money than the professional with large portfolio. At the beginning everyone earns less.
But the most important thing is to make your portfolio. Just look for some local non-profit organizations such as school or sport clubs and make a site for them. Or just make a few hobby projects and put them into internet. The site about breeding grasshoppers, or about West-African legends, or anything. You need to invest a little in hosting services, but you need to be able to show something that is running in Internet.