OK, then you have completely missed my instruction.
You must set the video mode BEFORE you import any of your pictures in Shotcut.
To be certain, select the 3:2 video mode, close Shotcut then re open Shotcut and THEN import your photos.
Vivi its very easy to fix this im havign the same problem.
Click on the image which has black bars. It has them because for some reason its very small,but its easy to fix that.
Click on it > Size And Position > DISTORT 1920:1080 (click anywhere on screen now to show how the img will really look) now after uve done this u already have no black bars 
Goodluck
Perhaps you did not read all of the thread because it is not a good solution to distort 3:2 ratio images to fit a 16:9 ratio view space.
The correct solution has already been revealed and once the OP fully understands the method described will solve the issue.
Hi
I have tried to set custum mode like you said, then closed and opened Shotcut.
The slideshow as shown in Shotcut does not have black side bars, but the finished video does - (smaller than before).
When I Export the video the aspect ratio sets itself to 1922 X 1282. I didn’t change this.
Vivi
Yes, change the export parameters to match the video mode.
Following the instructions I have proposed, I have zero issues. The result is a video complied from 3:2 ratio DSLR photos thus creating a 3:2 ratio video at the 1920x1282 pixel resolution, sans black bars.
My question to you is why do you want to create such an unconventional ratio video file which will not play on any modern TV, Monitor or mobile computer device at full screen without showing black areas left and right?
As suggested earlier on, prepare your images before importing into Shotcut and crop them all to 1080p.
what if i already edited my video and my photos?
should i delete everything i did, or is there another way?
my use case is: editing a conversation between 2 people in a podcast. while adding slides.the resulted video is 1928x996 (or something like this) and i want it to become 16x9
and i already edited the video into segments, with adding titles everywhere etc. i noticed the black bar later