I see that you are trying to use tagname inside the address field for variable tags.  In fact it's not at all allowed to use tags inside the address field except for OFSOPC tags which is a tag-based protocol and address and OPC communication works with tag-based addressing.
If you see an example IOdevice type is OPC that's why you can use tag based address.

I feel you have an IOdevice type as Modbus or modbus ethernet protocol and the addressing for modbus is like 40001 etc.. and you cannot use tags for that. That's why when you configure the registered address of modbus type you don't get an error.

Refer to Citect driver help which explains different protocols and their addressing formats.

In case if you don't want to use equipment-based addressing inside citect there is a way to disable it.
Go to the project editor -----> Tools --> options and uncheck "Disable equipment when populating tag list "  This will allow you to see simple tags configured from the project editor and there is no need to configure equipment etc.

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