I have an Aspire E15 E5-573G-531D laptop (with WIndows 10) and I was originally unable to boot from CD/DVD/USB stick, but I've now solved it and here's how ...
Obviously, first ensure you're using a DVD/CD/USB stick which is bootable (i.e. that you were able to boot from on a different computer).
Before you can boot from DVD/CD/USB stick, you need to access the BIOS. To do this, reboot the laptop and keep pressing F2 until BIOS is displayed. In BIOS, go to the boot menu and do several things:-
1. Put "Windows boot manager" low down the boot order (e.g. just before network boot).
2. Put whatever device you want to boot from high up the boot order (e.g. position 1).
3. Switch from “UEFI” to “Legacy” boot mode.
Now save changes and exit BIOS. When the laptop restarts, you'll get some DOS text with the GUID and MAC address and after a short wait it’ll say “exiting PXE ROM” and will then boot from the CD/DVD/USB stick .... Voila!
(Now do whatever you want to from the DVD/CD/USB stick as you want).
(NB: If you're running an app to Wipe unused space on HDD - if that's why you needed to boot from DVD/CD/USB stick - ensure you don't accidentally delete the boot partition(s) which are marked “Primary Unknown”. I deleted several of these partitions by mistake and couldn't restart the computer afterwards, so don't make that mistake).
Now, having run your DVD/CD/USB stick app, you'll want to put the laptop back the way it was so that'll it start Windows as before. To do this, first shut down (not reboot) the laptop. Wait for the lights to go off. Now switch computer on and keep pressing F2 until BIOS is displayed. In BIOS, go to the boot menu and:-
1. Switch from “Legacy” to "UEFI" boot mode.
2. Disable secure boot (It won't boot with secure boot enabled).
Now save changes and exit BIOS. It will now restart and run Windows as before. Voila!