It probably wasn't fully seated or the housing has debris in it. Let's get it back on securely.
First, find the side brush and the small screw that holds it in place. If you lost the screw, you'll need a replacement from iRobot. Clear any hair or debris from the brush's stem and from the hole in the robot's housing.
Now, line up the brush's stem with the hole and push it in. You should feel it seat. Take your small screwdriver and reinstall the screw to lock it in place. Give the brush a gentle spin with your hand to make sure it rotates freely.
If that didn't work, the housing itself might be damaged. In that case, you'd need to contact iRobot Customer Care for a replacement part. You can buy parts through the iRobot HOME App or their website.
Clean the side brush as part of your weekly maintenance to prevent future jams.