This worked out for me https://github.com/mageplaza/magento-2-shipping-method/issues/1
=> Changing "enable" for "active" in:
- /app/code/Mageplaza/Simpleshipping/Model/Carrier/Shipping.php 53
- /app/code/Mageplaza/Simpleshipping/etc/config.xml 6
- /app/code/Mageplaza/Simpleshipping/etc/adminhtml/system.xml 10
