If you are using Duplicati 2 as a service, you will not see the network share folder you see as a “normal” user as local drives in the local drive selection.
You can use the freeware tool RunAsSystem from Uwe Sieber:
https://www.uwe-sieber.de/misc_tools_e.html
Open cmd as admin and run the following commands as an example:
RunAsSystem "%windir%\System32\cmd.exe" /k dir
This will open a terminal as nt-autorität\system user.
At this terminal you can check the actual user with:
whoami
This will show: nt-autorität\system
With net use you can see if there is any network folder:
net use
With the following command as an example, you can bind the network folder for Duplicati running as a service
net use Z: \\fritz.box\FRITZ.NAS\EXT_HDD\Folder1 /persistent:yes
Now you should see the folder Z in your Duplicati 2 local folder selection.
Maybe there are other ways, but this works for me.