The following code does not compile:

FUNCTION foo
VAR_INPUT CONSTANT
   x : INT;
   y : INT;
   
END_VAR
VAR
   dd : ARRAY [0.. x*y-1] OF LREAL;
END_VAR

dd[0] := 1;

Are x and y not considered constants here?
I get the error : Border '((x * y) - 1)' of array is no constant value

Why do you declare the constant as INPUT? Makes no sense...

This will work:

FUNCTION foo
VAR CONSTANT
   x : INT := 1;
   y : INT := 2;
   
END_VAR
VAR
   dd : ARRAY [0.. x*y-1] OF LREAL;
END_VAR

If you want to create the array dynamic, you have to use __NEW if your target system supports this.

There is an option to use VAR_IN_OUT CONSTANT variables.
This is described in the Help of CODESYS

WARNING: Uncheck the option "Replace constants", otherwise you will get exceptions in CODESYS.
See the menu: Project => Project Settings => Compile options