Hello, I would like to know why I can't find a certain theme or other on the market of my forum?

Are you sure that your community and the theme you're looking for are compatible? It may be that the theme is outdated/not updated for the version you're running, or vice versa. Make sure you're on the latest version of Invision Community, and all available themes will appear in the Marketplace.

so there are very few free themes with the version I have, it's a shame

Can you share a link to themes you are trying to install 

9 minutes ago, sousou said:

so there are very few free themes with the version I have, it's a shame

Which version are you on, and are you not able to update to the most recent one?

I have the latest version

If I understand you correctly, the issue is that you're not able to find a free theme you like?

There are a total of 13 free themes, three of which are light themes and the remaining ten are dark themes. If you don't like any of these, then you'll either have to make your own theme (or edit the default theme) or purchase one of the paid themes.

As mentioned above, if there are no free themes you like, your options would be to either purchase a theme, or create your own theme. If you take a look at the following, this goes through options to get your site looking as you want