Hello @pragadeeshraju-2799,
I tested the following az cmdlet and it worked for the purpose
az webapp config storage-account add -g "your resource group name" --name "app service name" --storage-type AzureFiles --share-name "share name" --account-name "storage account name" --access-key "your storage account key" --mount-path "mount path in app service" --custom-id "your custom id" --verbose --debug
Added verbose and debug switches to see any issues in running the cmdlet
Please mark as "Accept the answer" if the above steps helps you. Others with similar issues can also follow the solution as per your suggestion
Regards,
Manu