Auto-Fill formula to end of column, not end of sheet?

The arguments to Cells are (row, column). They must be expressions that evaluate to numbers; you can't use "M" as a column indicator. Replace it with 13. Same for "N".

For Range, you have to use a string that gives a range, such as "M1:M1001" (there are other forms as well but let's just limit ourselves to your code). Anything that you put in there you would also be able to use in a worksheet formula like SUM. However, you are using the string "M2:LastRow_M", which makes no sense to Excel. When you put your variable name inside the quotes, it becomes part of a string rather than the value of the variable. You have to build the string using LastRow_M as a number. If you use

"M1:M" & LastRow_M

it will build a string that looks like

M1:M1001

See changes below.

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You appear to be trying to do the right thing in that you define and use LastRow_M and LastRow_N. However, you use LastRow when you set the value(s).

And don't use .Select ... Selection.; you should combine those lines.

This:

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Could be:

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Regards, TMS

Thanks for the rep.

TMShucks had a good catch that I missed. Just for completeness, second round of changes in blue:

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@6SJ: Well, I missed the whole Range("M2:M" & LastRow_M) bit so I think we're quits