I think it is just a typo on your part as the repository looks good. In your italicized lines below, there is /mainbionic/ all run together so you may have left out a blank space between main and bionic somewhere and the order looks weird too. Please go back to:
**System->Software Sources->Additional Repositories->Add a new repository ***
*
and check to make sure you typed in exactly:
deb https://cinelerra-gg.org/download/pkgs/mint19 bionic main
with a space before and between the words bionic and main. Try again and reply here if OK or not.