SUM: You cannot enter the downtime. You must start the uptime replication

One of my colleague encountered below issue while performing SAP support package stack using the Software Update Manager (SUM) tool.

Preparations for Downtime
Caution: The uptime replication is not running
You cannot enter the downtime. You must start the uptime replication

Solution

Login to your SAP shadow system, go to transaction code SM37. Look for the below CRR_OPERATION jobs.

Kill these jobs.

Now, go back to the SUM page, click on check status again, and click on Next. Ensure the job is completed successfully and you should be able to proceed to the next stage.