A vactrol is quite literally an LED or incandescent light bulb inside a closed space with a photoresistor.
When a signal (such as a gate) is passed through the light bulb, it begins to illuminate over a millisecond or so, causing the photoresistor to decrease its resistance to e.g. “open up” an envelope. When the signal (gate) stops, the light takes several milliseconds to “die down” to darkness, making the resistance gradually increase back to infinity. This results in an inexact, gradual envelope that can be combined with a VCA/LPG circuit to make musical, Buchla-y “plucks”.
Here’s an example of a DIY vactrol built from standard parts. (Source https://sound-au.com/project200.htm)
Here’s an 8-channel vactrol-based switched VCA/LPG that a friend made for me earlier this year:
Mutable Instruments Streams can emulate a vactrol in one of its modes (see its manual).
Email firstname.lastname@example.org with a full screenshot of the page where you see that error message.