ID not member of structure

In Ideal mode, you have specified a symbol that is not a structure member
name after the period (.) structure member operator. For example,

      IDEAL 
   STRUC DEMO 
      DB  ? 
   ENDS 
   COUNT     DW 0 
      mov ax,[(DEMO bx).COUNT]  ;COUNT not part of structure

You must follow the period with the name of a member that belongs to the
structure name that precedes the period.
This error often happens in conjunction with another error that caused the
expression parser to lose track of what you intended to do.