I have a question about a titan?

So I want to get a titan for my Astra Militarum army as some HEAVY support and on the app it says that I can have smth called a ‘Warbringer Nemesis Titan’ but it says it’s smth called a ‘Legions Imperialis’ unit and I’m just wondering if I could use that in 40K?

Sorry if this doesn’t fit here I didn’t know where else to post it😭😭😭