SQL Availability Group Validation Error
This isn't saying the default file locations need to be the same, this is saying that you don't have identical paths for all files in the database(s) on the secondary server.

Example:

Primary Replica: Database files are in E:\Mount1\, E:\Mount2\, F:\Mount3

Secondary Replica: Existing directories E:\Mount1\, F:\Mount3


You'll notice that the secondary replica is missing E:\Mount2\

The fix is to make the secondary have identical paths for the databases involved.