What is the reasoning why I cannot iterate thru Bit of Word in a word.

Code:
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.-------------------------------------+ FOR   32      x
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​​​​​​​.                                     +--------------------------
.
.         WORD:x <---------------  The qualifier for BitOfWord must be a constant.
.-----------] [----
.
.
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.-------------------------------------+ NEXT
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​​​​​​​.                                     +--------------------------