Error message examples
When the fourth parameter, «I», to Function Stepinterp is not specified, the array parameters to Stepinterp must have exactly one index in common. However, in this call, there is no common index between them. It is preferable to specify the index parameter, «I», explicitly.
You are calling a function that expects two arrays that have a common index. You have called the function by passing two arrays without any index in common.
Correct the arrays passed to the function to have a common index that you want the function to operate over, and preferably pass in the common index to the function as well.