Conditional Column in Dax not calculating as expected

Gdibbs , a couple of things:

1) M is case sensitive and spelling counts. You've written two different things: 

• [Plant]] = "AS5A" 
• "plant" is equal to "US5A"

Which option do you want? A or U?

2) It's difficult to see with your screenshots, as you haven't included the columns in one shot, but the example you did show does not appear to have any data < 1, so that will leave everything as blank (not null - if you want null you need to specify that in your column instead of the empty quotes "").

= Table.AddColumn(#"Filtered Rows", "Custom", each if [[Plant]] = "AS5A" and ([[#"Oct-21"]] < 1) then ([#"Oct-21"] * -1) else null)

Otherwise I can't see any obvious errors at a quick glance.