Are you sure it's less than 30 mins in length?
Not to sound like Mr. Obvious, but this is the most common reason why files aren't processed under the free trial account.
When you signed up for an account we gave you 30 minutes of free credit to upload audio/video files of your interviews, lectures, speeches etc. But we don’t automatically cut or edit your files, so any file longer than 30 minutes will be locked until you sign up to a subscription plan or join our Pay-As-You-Go plan. To get started, you'll need to trim your audio to 30 minutes or less. You can do this by using an audio editing platform such as Audacity (which is free).