There are a few things you can try if you are having trouble logging in to your Bill.com account from a mobile device.
First, check to make sure that your mobile device is accepting cookies from our site:
iPhone:

From the Home screen, choose Settings > Safari.
Tap Accept Cookies and choose "Never," "From visited," or "Always."

Android:

Open up the Browser and select Settings from the Menu
Select Privacy and Security.
Tap Accept Cookies.

Next, If your device is set to accept cookies from Bill.com, then clear them.
iPhone:

From the Home screen, choose Settings > Safari.
Tap Clear Cookies.

Android:

Open up the Browser and select Settings from the Menu.
Select Privacy and Security.
Tap Clear all cookie data.