I am getting an error when attempting to write to Firestore.
I am attempting to use a field containing the user uid for my security rule.
service cloud.f
resource.data
refers to the data already stored, so your rule allowed users to only update data that already includes their user ID.
What you probably want to check is request.resource.data
which is the new data coming in.
There's a rather extensive document about those fields and other security rules here: https://firebase.google.com/docs/firestore/security/rules-conditions