Hi, mate, thanks for playing. This one is supposed to be part of the standard Marla progression if you only interact with Marla (so no other events are triggered... no Lani romance, no meeting Miri, just Marla). that said, I've heard some other questions on this, so I feel like either the hint is too ambiguous or the scene isnt triggering properly. (Which version are you playing, btw? Usually these things wouldn't change, but all of the trigger points have been altered in the premium version)
The route: speak to Marla when available (avoid Lani). The turning points are when Liam asks about the rain, and when Marla explains how she's been caring for her plant experiment. If you've seen the conversations with Marla and Luna in the library (about the royals), you're on the right track.