I think one of the problems that I have and is related to this error is how to define ItemIDs in my OPC server that is visible by OPC Read/Write in Simulink.
How the PI Server tag names/Alias names/MDB aliases are mapped to OPC ItemIDs is explained the PI OPC DA/HDA Server user manual under "Principles of operation > ItemIDs, Browsing, and PI". In short, the way you build a fully qualified ItemID out of a PI tag has the following format:
For aliases, MDB, and pi Collectives please refer to the user manual.
Hope this helps.