Okay... it looks like there is no support and it's just self-help.
I finally managed to post my ad and I am able to identify the bug of the forum software.
When I first created my ad, I am not able to post even though it shows a success during submit. 3 times...
What I do is to remove the images and the ad was successfully posted.
I reply to the ad to add my 8 pictures and it failed.
"Errors
The following errors occurred with your submission
You have included a total of 8 images and/or videos in your message. The maximum number that you may include is 5. Please correct the problem and then continue again."
So I reply with 1st 5 images, then 2nd reply add the last 3 images.
So I deduce that the reason why I can't post my ad is because in my original ad, I got > 5 images. The bug is on submission, this error wasn't captured and brought up for me to correct it.
Posting here so that IF anyone encounters the same issue, you can try what I do and work around it.