You confirm the relationship after the coup. Near the smoke rising and the Salarian store owner.
You must wait for an invitation from her first to meet on the citadel. Restarting wont fix it because you haven't reached that part yet.
This.
"Comfirming" anything before the post-coup meeting at Meridian Place Market/Citadel does not count as locking-in.
Below are most if not all of the dialogue options for earning romance points with Liara (new and imported romance).
To enable lock-in at the post-coup meeting >= 4 points are required. (Needless to say, Shepard must not lock in anybody else nor choose "Let's just be friends." during their 1st convo in her Normandy cabin.)
+ 1 point
[default] You did the right thing. ("I was under house arrest. There wasn't much I could do to help you, anyway.")
[romance import] I missed you too. ("I would've liked that, but under the circumstances... I think I can forgive you.")
[default] I have a lot to live for. ("When there's so much at stake, I just think about what I'd lose if I fail.")
[romance import] I remember the ones I love. ("When there's so much at stake, I think about my friends, loved ones... what I'd lose if I failed.")
- Normandy / Liara's Cabin / 1st Meeting: "I'm sorry. I understand if you don't want to talk about it."
I appreciate you asking. ("The thought means a lot, Liara. Thanks.")
- Normandy / Liara's Cabin / Meeting After Tuchanka: "Sometimes I wonder how you do it."
[default] I think of what's at risk. ("We have a galaxy to fight for. The stakes don't get higher than that.")
[friendship] I have friends like you. ("I think of the people I care about. That helps keep me going.")
[romance import] It's all for you, of course. ("Every time the world's about to end, I think about how mad you'd get if I didn't stop it.")
- Normandy / Acquire Mission Ardat-Yakshi Monastery After Coup / VS Was Shot
Are you all right? ("Are you okay, Liara?")
- Normandy / Shep's Cabin / About The Time Capsule
I'd like her to decide. ("You know me well enough to fill in the blanks, Liara.")
So are you. ("You've been there for me, too, Liara.")
- Normandy / Liara's Cabin / After Thessia: "How many asari died because I demanded their help?"
[not taken interrupt] You did what you could. ("You couldn't have saved them, Liara, even if we got there before Cerberus.")
- Presidium Commons / Apollo's Cafe Before Coup / Acquire Barla Von's Mission
You should relax more. ("We'll get the people. Take a moment for yourself now and then.")
- Presidium Commons / Meridian Place Market After Coup / Lock-in Convo Before Thessia
Kids are kind of cute. ("Yeah, that's actually pretty cute.")
- Presidium Commons / Meridian Place Market After Coup / Lock-in Convo After Thessia
I would've liked that. ("I bet it was something special.")
+ 2 points
- Normandy / Liara's Cabin / After Thessia: "How many asari died because I demanded their help?"
Paragon Interrupt ("None.")
+ 4 points
- Normandy / Liara's Cabin / 1st Meeting: "Should we continue where we left off?"
[romance import] Absolutely. ("I'd like that more than anything, Liara.")
Source: Gibbed files
ME3 dialogue structure
ME3 conditionals
ME3 transitions