There is nothing wrong with that code, but it looks like prior to it, you are assigning a value to inv. I.e. you create a variable called 'inv', hence when you want to use the function, Matlab thinks you want to call the variable instead. Make sure you read the whole error, which should look like this:
'inv' appears to be both a function and a variable. If this is unintentional, use 'clear inv' to remove the variable 'inv' from the workspace.
To fix your code just change the name of the variable inv to something like inv_1or inv1. If you still have the problem, clear the workspace with: