Looks like with the last change in NMX in a Formula Editor is not any more possible to refer to Lists. So the only way is to type them in. It's very time consuming and error prone. Could you please have a look if it can be changed.
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It's a known bug.
Solved: Issue with formula editor :clicking on a list or l... - Anaplan Community
Workaround is: Either type manually or else switch back to Old Modelling Experience until bug is fixed
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