Try this if Columm doesn t otherwise if columm doesn t exist do that

Hi, I need help with a formula that replace null values to 0 if colum A and B exists , but often the columm B does not exist when  I do a columm to lines transformation. how would it be do write the try and otherwise to a columm that doesn t exists? I need this check at the begginning of the code because I have a lot of other code lines that need to know if the columm exists.