My photos are not selling. Could someone please give me some advice on how to improve the situation?

You need many more assets. If you have 10,000 plus assets, and they don't sell, you should worry about that.

Besides the tags that should be a lot more optimized and reordered, I would also pay closer attention to the titles of your assets as they do impact SEO results as well. Titles such as "Christmas ball" are way too vague to compete in an already oversaturated market and titles such as "Frisch zubereiteter Zitronensaft in einem Glas" do not mean anything to someone who's not German.

Nonetheless, having no sales from a 100 (or at least 80-90) unique designs does seem odd. I would suggest starting with a motif that you know quite well, maybe even at a professional level. This way you may be able to explore it in a deeper and more meaningful way, and you would also be able to craft titles and tags that are more descriptive, relevant, and targeted for your buyers.

You have a somewhat diverse portfolio; however quite a few of the images are in over-represented categories - lemons, oranges, pizza, coffee cups, business woman with crossed arms, hamburger, etc. And, of course, the seasonal images will languish until closer to the season they represent. 100 is a very small portfolio. You should be regularly submitting small batches, say 4-5 per day, every day if possible, to keep your account fresh and grow your portfolio steadily.

It's rumored that successful portfolios are typically between 500-5000 in demand assets before you start seeing consistent sales.

If you'd like to see what is in demand, try visiting the Artists Hub  and the Calls for Content.

A sale from only 100 photos would be possible but extremely rare. With 500 to 1000, you might start getting an occasional sale. Nothing is wrong. Just keep submitting. Also, those aren't photos. They are gen AI and many should be labeled as Illustrations, which I assume you're doing.