Maps.grf not found when generating maps using GRF Editor

[antonigaming]

Here's what I did.

I opened GRF Editor v1.8.2 >  Opened the GRF located at D:\ > Tools > Flat map maker > Under Generate maps, I checked the "use this grf" for the location of input maps then clicked "Generate maps"

It leads to an error saying FlatMapsMaker\OutputTextures\maps.grf not found