Set up CI with Travis for .NET Core - github

Question

I'm trying to set up a CI for my .NET Core 3.1 class library. I created an account at https://travis-ci.org/github and selected my repository containing the code for my class library for a CI build.
Travis is successfully watching my master branch for commits, but I cannot get the CI build to succeed.
My repository: https://github.com/MintPlayer/MintPlayer.AspNetCore.SitemapXml
Target framework: .NET Core 3.1
My latest .travis.yml file:
language: csharp
mono: none
dotnet: 3.1.302
script:
- dotnet restore
dist: trusty
sudo: required
History of attempted travis.yml files (all failed):
But somehow this always errors, this time with the following output:
Clean output:
docker stop/waiting
resolvconf stop/waiting
C# support for Travis-CI is community maintained.
Please open any issues at https://travis-ci.community/c/languages/37-category and cc #joshua-anderson #akoeplinger #nterry
Installing .NET Core
$ export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
$ export DOTNET_CLI_TELEMETRY_OPTOUT=1
E: Unable to locate package dotnet-sdk-3.1
E: Couldn't find any package by glob 'dotnet-sdk-3.1'
E: Couldn't find any package by regex 'dotnet-sdk-3.1'
The command "sudo apt-get install -qq dotnet-sdk-3.1=3.1.302*" failed and exited with 100 during .
Your build has been stopped.
Raw output: https://api.travis-ci.org/v3/job/709293291/log.txt
What's wrong with my travis.yml configuration. Why can't I CI test my class library with this config?
I mainly want to CI test for .NET Core. If mono could be included this would be very nice. All the examples you'd find in the git history are also examples that I digged up in other repositories.

Answer 1

You are using:
dist: trusty
That's Ubuntu 14.04.6 LTS which is not supported by .NET Core 3.1. Try a newer version of Ubuntu such as bionic.
This is also answered in the travis community post:
dotnet-sdk-3.0 is not available for Trusty (presumably because it’s EOL): https://packages.microsoft.com/ubuntu/14.04/prod/dists/trusty/main/binary-amd64/Packages Move to dist: xenial (which is the default so you can just omit it) or dist: bionic.