Ever tried to enter a feet-and-inches distance in a drawing and received the message "Point or option keyword required?" You probably thought, "Hey, I did enter a point!"
Don't worry, AutoCAD LT's not broken -- it's just not using the same units you are.
To check what you're using, head to the Drawing Units dialog. You can find it on the Application menu under Drawing Utilities, or by typing UNITS at the command line. I've talked about this dialog before, but I wanted to come back to how display settings affect input options.
The "Type" setting controls what AutoCAD LT will display and accept as valid units.
Depending on which display format is current, only some input formats are accepted. Architectural and Engineering accept numbers input in any format, but Decimal, Fractional, and Scientific only accept numbers in one of those three formats (i.e. nothing with a foot or inch mark will be recognized).
The other unique thing about Architectural and Engineering units is that their base unit is the inch. If you draw a line by typing 1'-6" for the length and then use the Distance command on it, it will display 18". For all other formats, the base unit can be whatever you want, from nanometers to light years.