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Formula is correct... it needs a number in D3.
It appears from your post that D3 contains something like 6 months and 2 days.
If that is the case then you need to extract the number of months using
LEFT(D3,FIND(" ",D3))
You then need to multiply it by 1 to convert to a number and use in your IF formula.
Complete formula is given below
=IF(F3="M","N",IF(LEFT(D3,FIND(" ",D3))*1<6,"N","Y"))