Fill Down not Working

I have a VBA macro that is supposed to add a formula to a cell and then fill it down the column to row 1001 so that it has 1000 instances. The formula itself is working fine, but the fill down portion is not working. The macro is set up to add a header to the last column on the right and then put the first formula in the second row of that column.

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This portions establishes what the last column to the right is.

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This portion inputs the formula.

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This portion is where I run into issues. When I run the code, the error message is 1004, AutoFill method of Range class failed. If I try to run the entire macro itself, it doesnt work, but if I run it line by line, it does seem to work. Any help would be greatly appreciated as I am still very new to VBA, but took on a bigger project. I tried to include the file, but it was too large to do so, so I included all of the code for the userform in hopes that it helps. Thank you guys!