Try any of the following:

=$I13 * LEN($A13)

=$I13 * ($A13<>"")

=AND($I13, LEN($A13)) 

=AND($I13, $A13<>"") 

If the formulas above return FALSE for any row and Column I is checked, then the two most likely explanations are:

1. Column A is not empty.
   You can test a range of cells to check if they’re empty using the following formula. Modify the range rng to fit your needs:
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   <code>=INDEX(LET(rng,A14:A20,

   IF(LEN(rng),

   ADDRESS(ROW(rng), COLUMN(rng), 4) & " not empty",)))

   </code>

   <code>=INDEX(LET(rng,A14:A20, IF(LEN(rng), ADDRESS(ROW(rng), COLUMN(rng), 4) & " not empty",))) </code>

   =INDEX(LET(rng,A14:A20,
      IF(LEN(rng),
        ADDRESS(ROW(rng), COLUMN(rng), 4) & " not empty",)))
   

2. Conditional format range mismatch
   If your conditional format custom formula is:
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   <code> =$I13 * LEN($A13)

   </code>

   <code> =$I13 * LEN($A13) </code>

    =$I13 * LEN($A13)
   

   The range of the rule should also start in Row 13. For example,

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   <code>=A13:I

   </code>

   <code>=A13:I </code>

   =A13:I