WHMCS v7.5.1 requires Ioncube Loader 10.1 or higher. The Automatic Updater verifies this requirement for you in v7.4.2. If you are using the Automatic Updater and are not yet on v7.4.2, then you will not see the v7.5.1 update but only v7.4.2. Use the Auto Updater to first update to v7.4.2. After this, v7.5.1 will viewable and the Automatic Updater will be able to verify all requirements before proceeding to this latest version.