This is kind of all over the place. I can't really tell what is going on. First I have one small corner of the live view showing where something looks cut off, but I have no context: the rest of the screen showing important parameters is missing. Then I have the print to file setup showing 18x24 page size. And finally a screen shot showing a 19.7 x 29 page size? Where did that come from? Where did you get a 19+ x 29+ page size? Basically I have no idea of your sequence and how you got to these results.
If you set up your print to file page size as 18x24 and you add a print at 18x24 (or smaller), there will be nothing cut off. If you add your border afterward, just make sure you don't use a B+ border which will expand the print size.
For me to see what is going on, I need a sequence showing what you are doing and I have to see the whole screen so I can see how it is set up. If the forum image size limitation is stopping you, just upload them to Google Drive or something and give a link. What would be even better is a screen recording/screen cast showing what you are doing in video form, but if you can't do that, I at least need to see the screen shot of the whole Qimage window where the border is getting cut off.
Bottom line: for anything to get cut off, either you've done something to make your print size bigger than 18x24... or your page size is smaller than 18x24.
Mike