The error message unfortunately is not helpful at all. It has nothing to do with the units.
The reason you get this error is because you are using the wrong type of index. You are using the vector/matrix index (which you get by typing [) but you should have used the literal index, the subscript which you can get by clicking on subscript in the math ribbon (section "style") and the keyboard shortcut is Ctrl and -
Normal you would get an error telling you that the variable h is unknown and you should get this error in case of q_a. But h is also a predefined constant (type h= to see which) with units and thats the reason for this misleading error message you got.

WE