This option has been removed in Locker 4.0+. Note that any files you already backed up will be automatically restored to the app when you open it. The app checks for any files that were stored in backup when it opens, but it is no longer possible to backup new files. We now recommend using your device's iCloud Backup feature to backup your entire device automatically.
You can also export your files from your Locker and save them however you want: https://float.support/article/9000205854
If you would like a refund of your original purchase, refunds must be requested from Apple Support, as the transaction is between the customer and Apple. Apple does not allow app developers to issue direct refunds. Request a refund from Apple here: Request a refund for apps or content that you bought from Apple