If cell B10 is blank, then return the value in D10

It should actually look like this: =IF(B10="",D10)

As for your main:
=IF(D10<K10,L10,IF(D10=K10,F10,IF(D10=0,0.00)))

Try this revision/re-sequenced:
=IF(D10="",0,IF(D10<K10,L10,F10))
which checks for D10 being blank/containing formula blanks
ahead of its subsequent comparison with values in other cell

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Max
Singapore
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