Country of Original or COO for your international orders is mandatory to comply with custom regulations.
You can specify the COO when creating shipments by clicking 'Electronic Custom Data' in the order section.
Other option is to enter the COO in your Selro inventory at the SKU level or Global settings level by following the below
1) Please go to Settings - > Shipping tab and enter the COO as a default value. This value will be used if there is no SKU level COO .
2) You can specify the COO at SKU level ( can upload a CSV file for all your SKUs). COO field is under shipping tab when editing the product.