Make sure when you are configuring filebrowser that in the "Storage Configuration" section you are adding an additional storage.
For mine I simply just added the path: /mnt/HDDs (as shown below)
Doing this worked and allowed the browser to launch!
I am quite new to TrueNas so this was just trial and error for me. Once the application was launch I was able to connect following the instructions on Installation - File Browser
I did run into an issue that for my sub datasets, specifically one for applications. It say I "don't have permission to access this." despite the user settings for all datasets being root. Not sure if anyone else would have some insight on this issue.