Hello,
Since React is not supported by Google, you may not find official Google documentation about it. However, based on other forum comments, I suspect you need to align the YAML file and build folder to be in the same path. For example, step 5 of this[1] tutorial mentions to create the YAML inside the build folder, and add some wildcard handlers (more about handlers here[2]). After this is done, deploying should work and align all resources. In addition, these[3][4] articles mention the same exact steps, with the latter as an automated script instead.
I hope you find this useful.
Sincerely,
Alexis
Google Cloud Platform Support