The missing handling time in Google Merchant Center feed processing errors is related to Shopping Actions. When advertising for shopping actions you need to list the handling time.
To resolve the issue, log in to Google Merchant Center and go to Shipping. Click on the service name you have listed, if you don’t have one listed create a new shipping setting.
Then scroll down to Delivery time and add your handling time. Then save in the bottom left corner.
Handling time is the difference between the order-cutoff time following placement of an order and when the shipment for the order is ready for transit. Use 0 if the shipment is ready on the same day as when the order was placed (if the order was placed before the cutoff time for the day).