I noticed that you are trying to create an array of transfer function variables. However, you have not declared the variable initially. This causes errors since the tf function gives a system model object as output.
Please refer to the section Array of Transfer Function Models in the given document on how to pre-allocate the transfer function array.
Also, you seem to have made syntactical errors in line 7 and 8 of your uploaded code, where you are trying to access the said tf array by using the variable k. However, k is a non-integer and cannot be used as an array index. Array indices must be positive integers.