Were those files and their folder made by Duplicati? Google Drive only lets Duplicati see files it made. There might be some online workaround, but a more sure-fire one for such a small backup is to move those files to somewhere without such odd rules (for example a local drive) and just run direct restore.
The best plan is to NOT create the folder by hand, because (as said earlier) Duplicati can not see the one that it didn’t create. It will offer to create it, and will be able because Google Drive allows duplicate names. This seems rather unusual, but it’s how they did it. You can web search on issues this causes.
So when setting up a backup, first make sure the folder isn’t there, then let Duplicati do folder creation.
If you have files you uploaded some other way besides Duplicati, there’s no way I’ve found yet to make them available to Duplicati using its default login scope, which is a restricted one that sees only its files.
Duplicati OAuth Handler run directly can (for now) also do a “Google Drive (full access) login” but it may disappear early next year because Google thought it’s insecure to give an app access to all of your files.
Sounds reasonable to me except for the transition plan. Sometimes you WANT to give an app files that it’s handling for you, and Google hasn’t provided any reasonable method that I’ve found yet to do that…
If you’re referring to OAuth 2 concepts such as documented in OAuth 2 Simplified, none of it applies to anything an ordinary user would do. The user usually clicks the AuthID button on UI Destination screen which should fill the web UI field automatically after a dialog with the service provider’s OAuth 2 server.
Each Storage Type has its own requirements that you need to fill in. For an S3 compatible backend, you need to specify a Bucket name, region and storage class. For other backends, like Google Drive or Microsoft OneDrive, you need to create an AuthID token to grant permission to Duplicati to get access to that backend.
Special purposes (or failure to auto-fill) can use the Duplicati OAuth Handler directly, then copy AuthID.
It’s more dangerous than the default “Google Drive (limited) login” which is why it’s not the default and why Google is trying to limit it (effectively killing it, except for app makers with time, money, and need).
Only you can decide if it’s too dangerous for you. Even if you fully trust an application creator to not be malicious, there’s still a chance malicious code could take over an application to do harm by its access.
I believe when you first go through the service provider’s OAuth dialog, it tells you what access app is asking for. For Duplicati, it’s probably file access, i.e. it doesn’t have or need access to all the services. OAuth 2 is somewhat safer this way. Duplicati never has your actual username or password, and what access is given is presented to you for your approval, though things might not work if you deny access.
After finding the app in “Third-party apps with account access”, you can get details by clicking on it, e.g.
and the date on the line below (which I omitted) matches when I created a limited access login. I later created an AuthID for full access login (and used it today to confirm it saw all files), but it’s not shown.