Sheet metal faces being non-planar is the most common issue that prevents an edge from being selected for an edge-flange. It is fairly common for cut-extrudes/boss-extrudes that are added and merged to sheet metal parts without the "Link to thickness" option selected, to cause faults the geometry. This occurs because SOLIDWORKS uses gauge tables, bend deduction, K-factors, etc. to properly simulate the dimensions of the part in both the flattened and formed states. These issues can be cumbersome and hard to track down.
I've found that the rollback bar can be a very powerful tool in troubleshooting these types of issues. You can rollback to a simpler state of the sheet metal part where the edge you are trying to add the flange to still exists.
If you can add an edge-flange in this rolled back state, we can use process of elimination to determine which feature(s) are cause the non-planar face condition, as seen above.
From here, the edge-flange can be tested with the rollback bar moved down below each feature one by one. Once the edge-flange can no longer be added, you know the feature right above the rollback bar is the root cause of the non-planar condition. Generally these issues are cut-extrudes without the "Normal Cut" option, boss-extrudes that aren't linked to the thickness of the sheet metal part, cuts that interfere with bend edges, boss-extrudes that fuse bends, and incorrect relief/ripping, to name a few.