"Grandpa friendly" Well, it is. You got it to boot from a USB-Stick. But before you get 'deep into the weeds' you might want to try "Friendly-Fossa" https://www.forum.puppylinux.com/viewto ... 902#p43902 --perhaps creating a USB-Stick the same way you did Fossa. Friendly-Fossa is even more 'grandpa friendly' in operation than Fossa.
Which doesn't solve your current problem. As other have written, we need more information about your setup.
That said, grub2config (AKA Grub2 bootloader config) works under almost all circumstances. Fossapup64 may have it on its Menu under Setup. If not, you can download it from here, https://www.forum.puppylinux.com/viewto ... 703#p29703. I recommend the following:
Boot up using your Fossapup64 on your USB-Stick.
Optional:
1) Download Friendly-Fossa from the above noted link.
2) Left-Click its ISO. A window to its contents will open. Leave it open.
3) At the bottom of your desktop you'll see icons for each partition on your computer. If you place your mouse-cursor on one you'll see a 'Tool-Tip' showing its size and format. Left-Click one to open a Window to it. [Remember you'll need a couple of GBs for your Puppy and Puppys --while they can be located anywhere-- have advantages if booted from a Linux formatted partition].
4. Right-Click an empty space in the above (step 3) window and from the pop-up menu select New>Directory. Give it a name, e.g. friendly.
5. Left-click the 'friendly' folder to open it. Leave it open.
6. From the opened friendly-fossa-ISO window, LEFT-PRESS, HOLD, then drag EACH of the following into the 'friendly' folder and select Copy from the pop-up menu: initrd.gz, vmlinuz and EVERY file ending in '.sfs'.
7. Left-Click the Friendly-Fossa.ISO again to close it.
Required:
8. Follow the above Steps to open Fossapup's ISO and compare its contents with the files you have on your hard-drive. If any of the files noted in Step 6 are missing, follow Step 6's instruction to copy it/them to the hard-drive. If others of Fossapup's system files are in a folder on the hard-drive, copy them into that folder.
9. If grub2config does not appear on your menu [Menu>Setup>Grub2 bootloader config] download it; left-click it to initiate installation and install it. Open Menu>Leave>Restart-X AKA Graphical Server. This causes Puppys to re-catalog what applications are available.
10. Run Menu>Setup>Grub2 bootloader config. It will search your entire computer for ALL operating systems and (over-writing your current boot-manager) create a new boot-manager using a new grub.cfg file. On reboot the new grub.cfg will offer to boot each of the operating systems that were found.