When trying to setup BitLocker, you get:
BitLocker Setup could not find target system drive. You may need to manually prepare your drive for BitLocker.
To fix the could not find target system drive error, you just need to free up some space on your hard drive, shrink the hard drive and re-run the setup again.
To free up some space, try uninstalling programs, getting a bigger hard drive, an additional hard drive, deleting patch install files, etc…
To shrink the drive, Right click My Computer -> Manage, under Storage, select Disk Management, right click your drive, click Shrink Volume and follow the wizard. (These are the instructions for Windows 7, shrinking drives in Windows 8 may be slightly different).