Why i cant report a scam?

When you dismiss that, it should say what you need to change to the right of the form.

In that screenshot, you're putting a URL in the steam name box. Use their name, not their URL. It should be telling you to the right of each field (in red) which field has restrictions you haven't met.

Click that X and then scroll up and see which box has a red error next to it.

After doing some experiments with people seeing the 100 characters or fewer problem, two new things to try:

• Try incognito mode
• Try clearing your cookies related to the SteamRep forums

Thanks to http://steamcommunity.com/id/Insane0000/ for helping me troubleshoot this.