I took the backup of my system on Time Machine disk , at that time it was running Mac Big Sur. Now i want to restore the data on Mac Capitan but its not Recognising the external hard drive.
Do i need to upgrade it to Big sur and then try restoration of data or it's possible on Capitan.
The disk is visible at Disk Utility section, but cannot be accessible.
Hope to see the solution soon.
Thank you in advance.
Your disk is probably formatted with APFS which wasn't fully supported until High Sierra. Time Machine itself used the HFS+ format until Catalina, only in Big Sur will Time Machine use APFS.
If the disk is APFS you need a High Sierra or new OS to mount the disk. Even having achieved that you may only be able to access the Time Machine backup manually. If the backup is a "remote" backup and is in a sparsebundle image you can double-click to mount it, if the extension is backupbundle change it to sparsebundle and it will mount.
Copying out your files is easy up to Catalina. Under Big Sur your backup disc may look empty as Apple has made entires invisible. To address that use the Terminal to move to the disc (in /Volumes) and you will probably find you can open the hidden folder back in the Finder (using open <folder> in Terminal).
HTH