Recently when deploying a new Kubernetes service I ran into this error...

 unable to find api field in struct Container for the json field "imagePullSecrets"

Nothing I could find online described what the issue was and it took a few days to realize the answer was simple.

I had put the imagePullSecrets inside the containers spec definition in the deployment. Instead it needs to be at the same level, directly inside the spec.

Once I moved the imagePullSecrets block up, everything started working again.