You should use MCE for that, this is from software.cisco.com
Version Upgrade using MCE New
Order major upgrades to software such as Unified Communications
You don't need a bootable image for upgrading through OS admin page, but as @Jaime Valencia mentioned make a new upgrade order through MCE.
AFIAK if you don't do it through MCE, the Cisco backed customer information will be on version 11 and it will effect us when renewing SWSS. I faced issues renewing the SWSSS with customer who didn't upgrade it through MCE/PUT.So open a upgrade request through MCE.
Upgrade readiness file is not the one which you mentioned.its ciscocm.preUpgradeCheck-00024.cop.sgn
Check your SFTP and make sure the CUCM can access the file from SFTP.
The cop file named ciscocm.preUpgradeCheck-00024.cop.sgn is in Sftp & CUCM read that file from SFTP storage. But after selecting this cop & above cops which I mentioned CUCM shows error: "A valid upgrade file must be provided before an upgrade can be started."