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FBD/LD Parser Error (FBDP0009): Invalid variable name '...'.

Two reasons are possible for this error:

Either an invalid identifier is used for a variable or an element of a structure or enumeration.

According to the IEC 61131 standard, naming conventions must be observed to ensure that each identifier can be resolved. Double-click the variable and correct it.

Further Info
For information on the naming conventions for identifiers please refer to the respective topics in the chapter "IEC 61131 Programming ‣ Variable/Port Properties
×‣ Extended Identifiers as IEC 61131-3 Variable Names
×".

Or, in case of an ARRAY/STRUCT/ENUM element, possibly the syntax for accessing this element within the user-defined data type is syntactically wrong (incomplete, etc.).

The respective object appears with a red background in the graphical worksheet.