Previously I've emailed Gramblr about this problem & they replied:
You can refresh the page and reboot Gramblr with the button on the top
left, then click again. Are you running the latest version of Gramblr?
I already did that even before I sent my email. Reboot Gramblr, refresh it, uninstalled & re-installed again. Still, the same problem. I can't upload the video.
Is there any specific requirement, or any limitation of the video spec i.e. size, length, type?
Thanks for any coming feedback.