You can’t declare your MIDI In blocks as variables to use in a Scriptlet
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A scriptlet is essentially just another plugin, and as such is unaware of other blocks and plugins in the rackspace—just like your Delay plugin knows nothing about your Reverb plugin.
It only processes the data it receives, it can’t discriminate where that data comes from. So that’s why you can’t declare your MIDI In blocks as variables to use in a scriptlet.
If you want to do that, you’ll want to make a rackspace script (or the Global rackspace script) instead of a scriptlet.
(source)