This is a small bug that can happen if you talk about Joint Projects in the embassy before meeting Sinem (in the
Milky Way). This bug is mostly harmless and disappears as soon as you meet Sinem.
If you keep on training with her, you will eventually reach a scene that ends with "
Dev Comment: This is as far as I've taken Sena's content for now." so it is difficult to know if the training will have a significant influence on the story.
And I thought that I had reported all typos!
You should download the full game from the links given in the OP. Downloading just the HTML page from the online version will not work. You would be missing about 4300 images, movies and audio files.
It is difficult to know why she does not appear because this depends a lot on where you are in the story and what you did before. For some events, you may have to wait around a week for her to show up. But if you tried to push things further when you met her for the second time (or third time, I don't remember) then she will not trust you anymore and you may have locked yourself out of her story. She will not come back until a future version of the game provides an alternative way to unlock her story.
In my current test game, when I met Harriet I gave her the ridiculously long name "Harriet/offw/Piper_mom" so that I don't forget that she is an offworlder and Piper's mom.

You should find Harriet in the kitchen one morning. If you assigned her to some shifts, talk to her before she starts working. I will not spoil the details, but basically you have to
flirt with Harriet during her shifts and then alternate between talking to Piper about her mom, talking to Harriet during her shifts, and waiting a bit for some events to happen.