The thing is that after the approval (submit) from Purchase Manager or Purchase Master Manager, is the Purchase User whom should send the email to Supplier, but the option isn’t available from dropdown Menu button.
I can see Email option after submitting the document.
I think your problems is related to the permission, please check the below image as it can help you:
It is because of workflow State, Purchase User is not allow for workflow state Submit or whatever state you have mention. You need to allow your purchase user for this state, then Email option will be available.