UNKNOWN_PROPERTY — Error Class PROPERTY, code UNKNOWN_PROPERTY

The object exists but does not have that property. Many BACnet properties are optional, and vendors implement different subsets.

Start free BACnet/IP → canonical

Diagnosis

When you hit this

Reading an optional property such as description or units on a minimal object implementation.
Reading priority-array on an object that is not commandable.
Assuming a property exists because a different vendor's equivalent object has it.

Resolution

How to fix it

Use ReadPropertyMultiple carefully
One unknown property can fail an entire RPM request on some devices. Split the request to isolate it.
Check the PICS
The vendor's Protocol Implementation Conformance Statement lists which optional properties are supported.

Normalization

What Forge does about it

A request that fails on some points and succeeds on others still normalizes. The tags that arrived resolve to canonical fields as usual; the ones that did not come back are listed in unresolved_tags and excluded from coverage_pct, so a partial read reports as partial instead of quietly looking complete.