Your dropbox is full and the amount of space you are using shown in the app or web interface does not correspond to the size of the Dropbox folder on your computer. You are not using selective syncing, i.e. all folders in the Dropbox cloud should be equal to the size of the Dropbox folder on your computer.
The reason might be you are using an external USB-drive on your computer.
When external USB-drives are connected, Dropbox asks if they should be backed up. If you selected yes, the content of the USB drive is copied to Dropbox cloud and occupies your space. If this was not the intention (for example if the external drive contains a backup copy of your files in the Dropbox folder), remove the external drive from the Dropbox folder.