Attempting to do a notifyBitbucket from a step with "agent none" or from the post/always block on a pipeline with "agent none" results in:

Required context class hudson.FilePath is missing
Perhaps you forgot to surround the code with a step that provides this, such as: node

While this can be worked around by wrapping in a node block, it doesn't appear that the plugin would really need the contents of the workspace in order to send a notification. It would be a great simplification if the step could be performed without a workspace.