I can't erase a disk as shown in the image.

The issue you're experiencing is likely because the disk is still attached to an instance in Amazon Lightsail. To resolve this problem and delete the disk, you need to follow these steps:

1. First, detach the disk from the instance it's currently attached to. You can do this through the Lightsail console.

2. Once the disk is detached, you should be able to delete it without any issues.

The error message you're seeing is a safeguard to prevent accidental deletion of disks that are still in use by instances. This is a common protection measure in cloud services to avoid data loss or system disruptions.

If you're still having trouble after detaching the disk, double-check that the detachment process completed successfully. Sometimes, it may take a few moments for the change to propagate through the system.

If the problem persists after detaching the disk and waiting for a short while, you may want to contact AWS Support for further assistance, as there could be an underlying issue that needs to be addressed by their technical team.
Sources
Resolve Lightsail disk attachment and usage issues - Amazon Lightsail