I really don't know what is happening to you, but the error says something about a file "opened by another process".
Try to copy the 3DS_Banner_Maker-Master folder to a short one like c:\BannerM and try to run from there.
For me, copying the exe file and the data file to t:\BannerM works fine:
View attachment 299842
Okay, It kinda worked, it doesn't show the error, but, I've tried to install the .cia in the 3DS and it shows an image, like, some default banner with beers and a text, lol
So it rlly didn't worked :'/
Also, it changed the "original 3 second sound" to the FBI one.
Seems that the "opened by another process" It's when banner and icons are in the "cbm_bannerImage", "ctpk_48x48icon", "ctpk_24x24icon" folders, guess it's opening the .png files in the background, even if it doesn't do anything with those