I couldn’t get strptime to work for me, either.
This is hacky, but you could use
{{states.calendar.work.attributes.start_time.split(’ ')[1]}}
That’s “split on spaces” to get a list, and then take 1 (zero based lists).
Templating is based on Jinja. Also, use the template dev tool (button at bottom of HASS sidebar) to debug templates.