lsb_release -a; # tested on guest
Description:	Debian GNU/Linux 11 (bullseye)
su - root
# adding the non-root default user "user" to the group vboxsf
usermod -aG vboxsf user
# then reboot system
reboot

lsb_release -a; # tested on host
Description: Debian GNU/Linux 10 (buster)
# if there are still access errors try on the host
chown -R ugo+r /path/to/shared/folder
admin