I had this issue with exporting and was easily able to solve it by physically creating an empty folder called “storage” on the web server in the folder wp-content > plugins > all-in-one-wp-migration. If you have FTP access (FileZilla, Dreamweaver, etc.), creating the folder will be easy. Then, just ensure that the folder permissions are set to 777. When you run the exporter (or importer), it should run without any issues.