Thanks for the reply . Try disabling antivirus temporarily and then export the key again.
Also once check registry permissions in regedit - navigate to key you want to export then right click key - permissions - check if administrators have full control.
If that doesn’t work then export in command prompt using below command
reg export HKEY_CURRENT_USER\Software\YourKey C:\path\to\outputfile.reg
Replace HKEY_CURRENT_USER\Software\YourKey with the path of the registry key you want to export.
Replace C:\path\to\outputfile.reg with the desired output path and filename for the exported file.
Let me know how it goes !