Unless the image is zoomed (some of it will be lost), it won't fill the space. An example of a ZOOM to the '6', then export at the YouTube 1:2 preset.

It's still unclear exactly how you want the 9:16 to display. Share the original, and an example (perhaps another image) of what the result should be.
Here's how...
Upload the file/s to a free server, such as Google Drive or MS OneDrive, get a shared (public) link, copy the link and paste it here or to me in a Private Message via the mail envelope in the top-right corner of this forum. When using Google Drive, if necessary change "restricted" to "anyone with link can view."