Explanation

Remove the update that causes the problem
* Take a snapshot of the OS disk of the affected VM as a backup.
* Attach the OS disk to a recovery VM.
* Once the OS disk is attached on the recovery VM, run diskmgmt.msc to open Disk Management, and ensure the attached disk is ONLINE.
* (Step 1) Open an elevated command prompt instance (Run as administrator). Run the following command to get the list of the update packages that are on the attached OS disk:
dism /image:<Attached OS disk>:\ /get-packages > c:\temp\Patch_level
* (Step 2) Open the C:\temp\Patch_level.txt file, and then read it from the bottom up. Locate the update that's in Install Pending or Uninstall Pending state.
* Remove the update that caused the problem:
dism /Image:<Attached OS disk>:\ /Remove-Package /PackageName:<PACK
* (Step 4) Detach the OS disk and recreate the VM. Then check whether the issue is resolved.
Reference:
https://docs.microsoft.com/en-us/troubleshoot/azure/virtual-machines/troubleshoot-stuck-updating-boot-error