Hi @Francesca
Welcome to the Knime Community!
Your question triggered me to look into this Column Expression node (which I never used before).
It’s possible to add these two columns with just one node (which was a nice surprise to me too).
I’ve constructed a small workflow: 25480.knar (10.4 KB) which looks like this.
The top table is the input, the bottom one is the result of the Column Expression node.
You have to add two expressions in the top half of the configuration, one expression for each column. In the bottom half you define the expression. By clicking on the expression in the top half, you will get its expression in the expression editor.
As you will see I moved the definitions of tempvar1 and tempvar2 to the else-branch, simply to avoid these calculations might go wrong (give an unwanted error) if either one of the columns is missing or the division should be done through zero.
Maybe that’s what caused the error you mention?
The second expression is not so fancy as the above one. It’s just an example… 
Kind regards