Theoretically, you don't need the "Bitcoin Install Dir" set... it's only required if Bitcoin Core is not installed in the "default" locations... Armory
should be able to function "fine" without it, it is just possible that it may not be able to
start Bitcoin Core for you automatically without it. No big deal, you can just start Bitcoin Core manually, then start Armory
However, you do need the "Bitcoin Home Dir" set correctly tho...
This is the directory where the Bitcoin Core "blocks" folder is... and should be set to the same value as the "datadir" shown in Bitcoin Core (in the "Window -> information" screen). Without that being set properly, Armory won't be able to find and parse the bitcoin blockchain data.
I'd suggest:
1. Shutdown Bitcoin Core (if running)
2. Leave "Bitcoin Install Dir" empty
3. Set "Bitcoin Home Dir" to: /home/barbara/snap/bitcoin-core/common/.bitcoin
4. Leave "Armory Database Dir" empty
5. Shutdown Armory
6. Restart Armory
If it is still not working, then post the logs using Pastebin as suggested by bob123