Use Purchase Saves to buy more saves for the app. Each time a file is saved, the save count is reduced by one. When the save count reaches zero, files cannot be saved and more saves need to be purchased.
Registering is another option for cross platform use, but store policies do not permit including additional information about that within the app.
If you cannot pay, you can uninstall and reinstall the app to get more saves. Be sure to export all of your files from the Home folder and any subfolders you created. Otherwise they are all deleted.