Welcome to the forums!
Unfortunately, we have no way to help if you don't provide more details. Are you using an email client (Outlook, Thunderbird, etc.)? Or are you using a webmail service? And which one: Gmail, Outlook, AOL, etc.?
Also, is there trash already in the Trash folder? If so, have you tried emptying that folder first (permanently deleting contents) and then tried moving messages there? (Some services, like Hushmail, I believe, have a small free storage and won't allow messages to be moved to the trash if it will exceed certain limits.) Or is your trash folder already empty or mostly empty)?