For apt to install a local file I think you need either a fully qualified path or to use "./" at the start for a relative path.

So "./$1/opensnitch_${1}_amd64.deb"

apt install 1.6.5/opensnitch_1.6.5_amd64.deb 1.6.5/python3-opensnitch-ui_1.6.5_all.deb

Edit: Here's a better example of what I think you would want:

#!/bin/bash
# Often good to assign a numbered parameter to a variable
VER="${1}"
apt install "./${VER}/opensnitch_${VER}_amd64.deb" "./${VER}/python3-opensnitch-ui_${VER}_all.deb"

Also - when debugging bash scripts it's often helpful to just put "echo" before the line you're questioning to see what exactly is being run. e.g.:

#!/bin/bash
VER="${1}"
echo apt install "./${VER}/opensnitch_${VER}_amd64.deb" "./${VER}/python3-opensnitch-ui_${VER}_all.deb"

That will show the the command that would have run rather than running it, then you can inspect it for errors and even copy/paste it to run it by hand.

I'd like to suggest that you take a different approach, though it looks like there is a workable suggestion already.

Consider using apt_preferences to pin your versions instead of scripting.

https://wiki.debian.org/AptConfiguration#apt_preferences_.28APT_pinning

Here's a clip from one of my distrobox builds:

Package: python3.10
Pin: version 3.10.*
Pin-Priority: 999

Is this installing a local .deb file or installing from a repo? If installing from a repo, the .deb and the full file path are unnecessary. If you're installing a downloaded file, use dpkg -i package.deb not apt.