Open the files below
Pro/Backup/Ajax/Listing.php
Pro/Backup/Ajax/FileList.php
and on or around line 55 change this line:
$isValidLicenseKey = (new SiteInfo())->isStagingSite() || (new Licensing())->isValidOrExpiredLicenseKey();
to:
$isValidLicenseKey = true;
disclaimer: for testing purposes only.