Getting access denied from CapRover deploy

I am attempting to do a deploy of a sample application in a Droplet using CapRover and I am getting:

Error: {“message”:“pull access denied for microsoft/aspnetcore-build, repository does not exist or may require ‘docker login’: denied: requested access to the resource is denied”}

I am using the instructions in Getting Started · CapRover. I created a CapRover Droplet. I got to Step 5: Deploy the Test App. I downloaded the asp_dot_net tar file to my local system then transferred it to the droplet. I extracted the tar then tried the caprover deploy but when it asked for a branch it would not take master and I could not make it happy. So I tried caprover deploy with the tar file specified but I get:

Error: {“message”:“pull access denied for microsoft/aspnetcore-build, repository does not exist or may require ‘docker login’: denied: requested access to the resource is denied”}

I did a Docker login but either I did that wrong or that is not the problem.

I have dotnet installed.

I am new to many of these things; Linux, Docker, CapRover and Droplets especially. I am using that demo project just to become familiar with the way things work, of course. I do not know what to learn to solve the problem. Does anyone have at least a clue about the problem?