Hi Eli,

This simply says the data storage allocated for your organization has been exceeded. You have to delete records and free up some space or purchase additional space.

In Salesforce you have 2 storage categories

1) File Storage - This constitutes all the files/attachments that are uploaded in your org.
2) Data Storage - This is where your standard and custom object data resides.

Developer edition usually has very low data storage limits (5 Mb which roughly is 2.5k records (2kB -per record)). So you could delete some of the data (std/custom records) in your org and then you will be able to create new records.
You can check the storage limits by going to :

SetUp --> Administer -> Data Management -> storage Usage

If you can confirm some records to be deleted, the Mass delete record feature will helpful for you. You can find it at Data Management > Mass Delete Records under Administration Setup (You should have required permission). There may be lots of Task and Event records hopefully where you no need. You can delete them and free up some storage.

You can also check this free app for storage help:https://appexchange.salesforce.com/listingDetail?listingId=a0N3000000B5fvwEAB

Have you verified this article,

https://help.salesforce.com/articleView?id=000349542&language=en_US&mode=1&type=1

For more detail, refer this example :- 
https://developer.salesforce.com/forums/?id=9060G0000005UsnQAE

I hope you find the above solution helpful. If it does, please mark as Best Answer to help others too.

Thanks.