"'default' key contained unknown value"

Server log

[ERROR]: Permission node 'myplugin.use' in plugin description file for MyPlugin v1.0 is invalid
java.lang.IllegalArgumentException: 'default' key contained unknown value

What it means

The plugin still loads, but this one permission is skipped and will not exist on the server.

What causes it

The default of a permission is not one of the allowed values: true, false, op, not op (also written notop, !op, admin, isop and a few other spellings).

How to fix it

Before

permissions:
  myplugin.use:
    default: everyone

After

permissions:
  myplugin.use:
    default: true

yes and no also work, because plugin.yml uses YAML 1.1 where they mean true and false.

Check the whole file. Paste your plugin.yml or drop the plugin jar into Permistry. It finds this and about 40 other problems, and explains each one in plain words. It runs in your browser and nothing is uploaded.

Open the checker