。
I'm free tomorrow morning.
Literal
Tomorrow [possessive-の] morning [if/as for-なら], am-open.
なら is a conditional that also functions as a topic-restrictor — 'if we're talking about X, then Y' or 'as for X.' Here it carries a contextual nuance: 'if the question is tomorrow morning, I'm free.' Implied is that other times may not be free. 空いている ('is open') is the progressive state of 空く ('to become open/vacant').