I keep getting this error message even though I have nothing in my account.

Hi Simon 

Thanks for reaching out in the Lucidchart Community! 

Images generally take up the most space in your account – if your storage is full and you have no documents this is likely your case. To verify that you can check how much of your storage is being used and what it is being used by (images and/or documents) in your Account Settings at the bottom of the left panel. If you hover over each of the colored sections you can see what that portion of the storage represents.

As a workaround to free up storage I would recommend that you clear out as many images as you can from both your documents and your Image Manager. You can access your Image Manager by clicking the image icon next to the magnifying glass above your shape library. 

I hope this helps! Let me know if you have any further questions.