Windows 7 doesn't support VHDX file format. This is only supported in Windows 8(.1) and Windows 10. For Windows 7, you need to convert the VHDX to VHD.
Solution - A) BEFORE you use Disk Manager, go to the respective drive where the backup is stored and double click on each VHDX file you want to review.
Step 1: To mount VHD, right-click on This PC >> Manage >>Disk Management and click Attach VHD. right click on This PC then Manage then Disk ...