I have applied IF to 5 cells that work but one wont

1st, you have a circular reference error in your sheet - that is when a formula in a cell, references the cell it resides in.
A quick check tells me that on sheet Overview, you have
K10=SUM(K4:K10)
As you can see, K10 is referencing itself in that formula. It should be
=SUM(K4:K9)
The same is happening in at least L10

2nd, you would be far better served if you have ALL data on 1 sheet, rather that a sheet per month. This will make all summaries much simpler.

3rd, please show some sample data, and expected outcome, so we know what you want