Subscribe to one of the Creative Cloud packages (Photography is most likely your best bet): you only get LR Mobile with LR CC.
thank you so much, you have no idea how long i have had that question.
Pity you didn't ask earlier. It's all explained in the Lightroom Mobile FAQ
The Lightroom iPad, iPhone, and Android phone apps are included as part of the following Creative Cloud membership plans:
You can use the mobile apps for free with a 30-day trial, but to keep access, you will need a Creative Cloud membership plan with Lightroom 5.4 or later or Lightroom CC.