Google for WooCommerce

There is no attribute to be able to set the boolean for Identifier Exists attribute option. This is useful for products without GTINs, MPNs & Brands (You can still declare a Brand and have identifier exists as false though, which sets GTINs, MPNs to false).

https://support.google.com/merchants/answer/6324478?sjid=4170186885114842624-NC#zippy=products-without-gtins-mpns-brands

Take a look under:
Examples
– Products without GTINs, MPNs & brands

This is used for handmade or one of a kind goods, and needs to have a Boolean value (true, false) in order to bypass the GTIN, MPN attributes (setting [identifier_exists] = false).

From Google Merchant Center Help:

How to fix your issue
For each product, provide the appropriate unique product identifier values (GTIN [gtin], MPN [mpn], and brand [brand]) based on our UPI guidelines. You may also need to add a value for the identifier exists [identifier_exists] attribute.

If there is an option for setting [identifier_exists] attribute, could you please specify where to find and use it??