@aymkdn I just came to the Community Forum earlier with the same problem & (I think) I found a solution:
It could be a problem with the Googlesheet Node version under the current N8N version. The solution ( according to @fxholl is mentioned here : Update Google Sheets node: Could not find column for key) Basically use the old Googlesheets Node instead of the new one - worked for me 
From the screenshot @treyr has provided I believe It’s also the old GS Node (?)