I've tried to change the ID name but it did not work.
1 exception(s):
Exception #0 (InvalidArgumentException): Element with id "system_cron_template_use_separate_process" already exists
Exception #0 (InvalidArgumentException): Element with id "system_cron_template_use_separate_process" already exists
#0 /chroot/home/this/my.com/html/vendor/magento/framework/Data/Form/Element/AbstractElement.php(93): Magento\Framework\Data\Form->checkElementId('system_cron_tem...')
#1 /chroot/home/this/my.com/html/vendor/magento/framework/Data/Form/AbstractForm.php(157): Magento\Framework\Data\Form\Element\AbstractElement->addElement(Object(Magento\Framework\Data\Form\Element\Select), false)
#2 /chroot/home/this/my.com/html/vendor/magento/framework/Data/Form/Element/Fieldset.php(212): Magento\Framework\Data\Form\AbstractForm->addField('system_cron_tem...', 'select', Array, false)
#3 /chroot/home/this/my.com/html/vendor/magento/module-config/Block/System/Config/Form.php(392): Magento\Framework\Data\Form\Element\Fieldset->addField('system_cron_tem...', 'select', Array)
I tried changing the ID here: vendor/magento/module-cron/etc/adminhtml/system.xml
It did work.