That happens when your IF function doesn't have a FALSE option to
return. In an IF, you provide a logical test, and tell Excel what to
return when that logical test is TRUE and what to return when that
logical test is FALSE. Your IF tells Excel to return the value in C3
when the logical test is TRUE but there is no instruction for FALSE, so
Excel simply returns the word FALSE.

As a fix, you need to provide Excel an instruction for what to do when
the argument is FALSE.