Hello,
The behavior is expected. First of all, the warning about resolving value references means that the template you specified in the action configuration resulted into an empty value upon execution. As you are to grant mailbox permissions, the template must resolve into a valid distinguished name. the following tutorial should be helpful: https://www.adaxes.com/help/AutomateExchangeMailboxConfiguration.
As for the The mailbox is not ready message and the changes not applying, it happens because the changes are made before the mailbox conversion to shared. To achieve the desired, you need to order the actions so that first the mailbox is converted and only then its settings are updated.