Send email action is not triggered on the entry trigger point of a workflow state.
So in order to work the send email action, it requires an after-submit trigger.
If possible add the button action in the previous state, so that on the button click an after-submit trigger occurs, and in the next state, the send email will work even if the trigger point is an entry.
If two states are there and the transition to the 2nd state is normal(not on a button click),then the send email in the 2nd state will not work on the entry trigger point.