Maxscale error - but there is no valid alternative to swap to

Maxscale was unable to promote potential slave node as master,

Do we now why it said no valid alternative to swap to,

The current master server ‘server01’ is no longer valid because it is in read-only mode, but there is no valid alternative to swap to.

I am looking to understand why server02 it didnt promote even though it was a proper slave./potential master.