I(t-Ts)= co*n(t) +c1* n(t-Ts)
I'm guessing that t has a value of 1 at some point and ts=1 nanosecond, so (t-ts)=0 at some point. As the error message indicates, the array indices must be positive values (or logical values).
Other possibilities are that (t-ts) results in a positive non-integer value or a negative value, both of which cannot be used as indices.
Note that this index problem happens with I(t-Ts) and with n(t-Ts). Without knowing more about what you're doing, I can't provide a more specific solution.