Alphanumeric column error

Good morning pooramit4 ,

To cater for your data being numeric and alphanumeric I've added an initial conversion to text - Text.From(_), I finish by converting text values back to numeric - Value.FromText(...). If you add the following step to your Power Query query your data it will produce an output of numeric dates. Replace #"Previous Step" with the name of your previous step.

= Table.TransformColumns(
  #"Previous Step", 
  {{"Duration", 
    each if Text.StartsWith(Text.From(_),"YTD") 
    then Value.FromText("20" & Text.Range(_,7,2) & Text.Range(_,4,2)) 
    else _, type text
  }})

 This...

gives...

Hope this helps.