I successfully worked around the issue by updating the /Library/Application Support/Perceptive Automation/Indigo 2022.1/Plugins/MySensors.indigoPlugin/Contents/Server Plugin
/plugin.py script to recognize the internal 21 code.
Here's the diff:

34c134,135
< "GATEWAY_READY" : [14, "Gateway ready", ""]

"GATEWAY_READY"             : [14, "Gateway ready",             ""],
"DISCOVER_RESPONSE"         : [21, "Discover response",             ""]

715a717,719

        elif itemType == self.getInternalNumber("DISCOVER_RESPONSE"):
            # 21 Ignore
            pass

P.S. Indigo 2022.1 is complaining about this plugin that it won't be supported in future releases. Is there anyone out there planning to update this plugin?

@yury Yes

I am getting this error about every 20 minutes from my gateway and it appears to be happening to all of my sensors. I am running all of my devices including the USB gateway on MySensors library 2.3.2. The gateway is connected to a Perceptive Automation Indigo home controller. Any ideas? A snip of the log is below:
2022-11-26 16:38:11.012 MySensors Error received '17:255' NoneNone0 failed: item type not found
2022-11-26 17:18:06.844 MySensors Error received '60:255' NoneNone0 failed: item type not found
2022-11-26 17:18:11.917 MySensors Error received '100:255' NoneNone0 failed: item type not found
2022-11-26 17:18:16.992 MySensors Error received '17:255' NoneNone0 failed: item type not found