Your function is defined to accept and use two input arguments.
function SEK_ut = SEKDrive(x,y)
You are only specifying one of those input arguments when you call it. [I'm assuming either that your SEKDrive function is defined in a function SEKdrive.m or you're actually calling it in your code as SEKDrive. The case of function and function file names matters in MATLAB.]
What value or values should MATLAB use when the commands inside SEKDrive try to operate on y?
It's as though you asked me "I want you to add two numbers. The first is 3. What's the total?" I don't have enough information to answer the question, and when MATLAB encounters that situation often its response is an error like:
Error using +
Not enough input arguments.
To correct the problem, specify a value or a variable for you as the second input when you call SEKDrive or modify the SEKDrive function so it only accepts and uses one input argument.