Alerts are available for Private Last Mile, but are NOT available for Last Mile testing.
Private Last Mile clients are deployed and implemented by customer sites directly. As such, the hosting machine performance can be considered under the control of the customer, and the data is presumed to be relatively "clean". Alerts will be meaningful to the customer.
Last Mile clients are deployed and owned by a vast array of end users, which (as intended) are outside full control of Dynatrace Synthetic Monitoring as it pertains to the hosting machines' performance. This variable factor means that Synthetic Monitoring may not be able to resolve the data. The data is considered "dirty" and alerts cannot be implemented in a meaningful manner.