What error you're getting exactly?
I will reiterate the steps just in case:-
1. Take the screenshot of your screen using a light shot or
using your mobile phone or built-in feature using the Print Screen or If you're on
Windows 10 then you can even take one using Windows + Shift +S on your keyboard.
2. Then you can choose directly to either paste it on your post while pressing
Ctrl + V or If you've saved it on your windows then you can click on "
Attach files"
3. That should open a dialogue box and you can select the screenshot by going to the directory where you've saved it.
4. Select the screenshot and then click on "
OPEN" and it should upload the video here.
Second Method:-
Click on the following symbol or logo, which you will see while making a post on the forum.
Then you will get few options, either you can drag and drop the image there without browsing your computer for its location or you can upload directly by entering the URL or just use browse feature there by clicking on the upload icon on the top left.
Hope this helps.
Regards,
Nody