More importantly... what are you seeing in the Test Result view? You should be seeing something like this:

Let's isolate the problem!
- The JUnit reporter setup in
wdio.conf.js file;
- Post-build Actions setup for the JUnit reporter (in Jenkins);
- Verify the build
Workspace (in Jenkins, post test-run);
- Others
1.
( My JUnit Reporter wdio.conf.js config )
reporters: ['junit'],
reporterOptions: {
'junit': {
outputDir: './reports',
outputFileFormat: function(opts) {
return `results-webdriverio.${process.env.USER}.${opts.cid}.xml`;
}
}
}
2.
( My setup for JUnit Reporter )

Most likely you have an issue with the path to the JUnit .xml. Use relative paths! (e.g: reports/*.xml, reports/**/*.xml, etc.).
Note: Basedir of the fileset is the workspace root.
3.
Check your build Workspace and see if the .xml report has indeed been generated and is residing in your path (e.g: in the /reports folder).
If not, then I'd go into the Console section and start looking into your build logs to see where the issue is.
!Note: I get the same Jenkins input errors (red notifications), but as far as I know, I have been always getting them. The .xml reports still get evaluated by the JUnit plugin.