sharepoint online – setting a column value in shareoint validation

I want to do an IF statement and if that's true, then I want another column to be defined. I'm using list validation on a Sharepoint list:

= IF ([Master vs Copy]) = "Master", ([Duplicate Rationale]) = "None), TRUE) 

For example, the column labeled "Duplicate Justification" is a column of choice and I want to set it to None (which is in the columns).

Any help would be brilliant because I do not know how to define a column in the list validation.