Debian disables unprivileged user namespaces by default, and your "issue" is consequence of this. Basically I see 3 options:
1) You do what error message says and make sure that /opt/Caprine/chrome-sandbox is owned by root and has mode 4755 (installation I made for the sake of tasting was indeed owned by root, but had got wrong permissions) ;
2) You enables unprivileged user namespaces all together via sysctl kernel.unprivileged_userns_clone=1 ;
3) You run APTus and you install sparky (or liquorix ) kernel .