The help for the "node label" field states as its example:

"label_1,label_2"

This example will cause the following tooltip:

"There’s no slave/cloud that matches this assignment. Did you mean ‘label_1’ instead of ‘label_1,label_2’?"

I believe this is caused by a bug on https://github.com/jenkinsci/elastic-axis-plugin/blob/master/src/main/java/org/jenkinsci/plugins/elasticaxisplugin/ElasticAxis.java#L104 - the form validation is tokenized on " " not ","