I can't summon during the quest 'engaging the empire'
Solution 1:
The ability to summon (in this case Ramuh), is available when Noctis is not being attacked.
You'll have to move to a safe-ish area not too far from the action and activate the summon. You'll know a summon is available because the button to do so appears on the lower left of the screen under the Technique Gauge.