eurocallreturn(strike, price, n_iterations);
That line calls the function to compute a value, and then discards the value because the semi-colon says not to display what was returned.
avereturn = mean(eurocallreturn);
And there is a second call to eurocallreturn, this one without arguments -- not enough input arguments.
Perhaps
euroreturns = eurocallreturn(strike, price, n_iterations);
avereturn = mean(euroreturns);