Forcing the boot menu to wait for manual selection

In Debian 10 modifying the "GRUB_TIMEOUT" parameter to -1 and then running "update-grub" leads to an error and the boot menu has the same behavior. See below.

Code:1) Uncommented part of /etc/default/grub

GRUB_DEFAULT=0
GRUB_TIMEOUT=-1
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX=""

2) Content of script /usr/sbin/update-grub

#!/bin/sh
set -e
exec grub-mkconfig -o /boot/grub/grub.cfg "$@"

3) As root in /usr/sbin

ls -l *grub*
-rwxr-xr-x 1 root root 1041064 Mar 1 23:50 grub-install
-rwxr-xr-x 1 root root 804664 Mar 1 23:50 grub-macbless
-rwxr-xr-x 1 root root 8023 Mar 1 23:50 grub-mkconfig
-rwxr-xr-x 1 root root 217528 Mar 1 23:50 grub-mkdevicemap
-rwxr-xr-x 1 root root 821368 Mar 1 23:50 grub-probe
-rwxr-xr-x 1 root root 4112 Mar 1 23:50 grub-reboot
-rwxr-xr-x 1 root root 3562 Mar 1 23:50 grub-set-default
-rwxr-xr-x 1 root root 64 Feb 27 16:07 update-grub
lrwxrwxrwx 1 root root 11 Mar 1 23:50 update-grub2 -> update-grub

./update-grub
./update-grub: 3: exec: grub-mkconfig: not found
What am I missing?