"Why wont this run? "Array indices must be positive integers or logical values."
The clue is in the error message. Array indices... so you must have some indexing somewhere. Look at your equation:
.. C(((R1^2)*C-L)/((R2^2)*C-L))) ..
yep, you've got C(something). Looking at the something, it's very unlikely to be an integer index. Considering that C is scalar, it's very unlikely you meant to index C. Maybe you're missing an operator?
To make your expression more readable, I would recommend spacing out some terms. I would also recommend against using brackets when they're not needed:
f = 1/(2*pi) * L*C ?? sqrt((R1^2*C - L) / (R2^2*C - L));