Recent Stretch 9.8.0 Gnome DVD, via Transmission (torrent) for my GF's old ASUS laptop: I noted that my Verbatim DVD disks are intended for data/media, not booting. (These disks have a movie film reel illustration on the front of the DVDs.)
Previously, I had noted your problem with the USB flashdrive too--in tandem with another such Verbatim DVD. Actually, that image on my Verbatim DVD then was supposed to be limited to net install (netinst) for a CD. (I was in a hurry and couldn't locate my CDs.) I got a message that the "CD" was invalid. Anyway, the disk wouldn't boot.
I had tried previously to use the a 32-GB Lexar thumbdrive to net install, also--unsucessfully. The DVD was invalid, and couldn't work, either. Nonetheless, the Lexar and the Verbatim disk together did boot, and did perform--to the point of package install. The pair "needed" each other to function--seemingly almost succeeding. (The warning message indicated otherwise, anyway.)
The next day, I tried using another Verbatim DVD, with the current Gnome image for DVD--still no dice--similar failure. After all, this was now a DVD iso burned onto a DVD, so I became a little frantic. During install with this Verbatim disk, I noted also that I couldn't format the / (root) partition to bootable with the EXT4 file system. I had never come upon this problem before with Linux.
I then tried a Philips DVD-R (burned with same Gnome image as this latter Verbatim, burned just a few minutes before), and was then able to install that. Thus, I was able to format the / partition correctly as bootable beforehand in installation, too. The filesize of the iso and number of files on both disks were identical.
Aside: It seems that with my GF's old ASUS dual-core Win 8/8.1 "Best Buy special" laptop, Debian should be installed with Gnome. No GUI becomes evident with an older Stretch XFCE 9.7 CD I had tried at first--used successfully before with my desktops. Merely lowly user "$" command prompts pop up. I had tried this also before with Jessie (Debian 8)--again without success. I was "shepherded" then into Gnome, instead of XFCE.
Moral: I made the mistake of using a DVD instead of a CD, for a CD image--and also using unsuitable media--which seems intended basically for data and entertainment content, not booting isos. It seems to me here that the media used does matter: Beware! (I need to learn to use thumbdrives correctly for this install function, also.)