You are using the assign-plus-pop so the Capytalk expression stored in c has a side-effect but no value. But the problem generating that error message is that you cannot send triggerEvery: to a Sound; you can only send it to an EventValue. To convert a Sound to an EventValue, you can send it the message L (to get its left output) or R (to get its right output).
The following version of your Script works:
| c |
c := ClockStream sound L triggerEvery: 2.
(inputs at: 1) start: 0 s index: c.