Your code attempts to call your prime function with 1 output and 0 inputs and use whatever it returns as the first input to ode15i. But as written your prime function requires 1 input. You need to pass a function handle to your prime function into ode15i.
ode15i(@prime, ...
However, this still won't work but for a different reason. As stated on the ode15i documentation page, the function handle input to the solver "must accept the three inputs for t, y, and yp even if one of the inputs is not used in the function." Your prime function only accepts 1 input, which plays the role of t (using the terminology from the documentation page.) It needs to accept y and yp as well.
I also strongly recommend you not call load in your prime function. That will cause MATLAB to load the MAT-file each and every time ode15i calls your function, which likely will take a lot of time. Instead load the MAT-file once before calling ode15i and pass whatever additional parameters prime needs into it using the techniques from this documentation page that is also linked from the documentation for the odefun input argument on the ode15i documentation page.
