I have inherited the maintenance of a Magento 22.214.171.124 site that currently has Onepage Checkout enabled. The owner wants to remove it, but when I disable, the Proceed to Checkout buttons disappear from the Cart page. I have searched around and haven't found any solutions that fix my problem.
I met same problem and solved. My site installed a onestep checkout extension with My Theme and I disable the onestep checkout in the Theme's config and enable Onepage Checkout in Configuration. Hope it can help you. It should not be a final resolution.