Error trying to get ubuntu via Nexus Docker Proxy registry

Set ubuntu:18.04 to FROM of Dockerfile.
When executing the docker build pipeline, it ends with an error at the FROM part.

■ Error details
Error response from daemon: manifest for not found: manifest unknown: manifest unknown

■ Confirmed
Confirm that an error does not occur when not going through the Nexus Docker Proxy registry.

Similar error occurs when docker pull in pipeline or local environment.

Could you please let me know if you know the cause and how to fix it?