​@Nano a user posted a similar thread over here with a solution:

Hope that helps!

Hi ​@Nano , 

Thanks for posting in the Lucid Community! Purchasing extra storage is not an option within Lucid. You can, however, upgrade your account to one with more storage. You can find the details for each account level and consider if upgrading might be a good option for you in this help center article.

Here are a couple of specific things to look at when working with storage:

• You can check how much of your storage is being used and what it is being used by your Account Settings under the "Plan and Billing" tab. If you hover over each of the colored sections, you can see what that portion of the storage represents.
• Images generally take up the most space in your account, if you are running out of room it is recommended that you clear out as many images as you can from both your documents/boards and your Image Manager. You can access your Image Manager from the file menu at the top of your diagram editor in Lucidchart (File> Settings> Image manager).
• Please note, when deleting documents, in order to free up storage space you will need to navigate to your trash folder and select to delete the documents from your trash as well.

Please let me know if you have any additional questions!