"Invalid: Static Encounter shiny mismatch" is not the same error as PID Mismatch. That error indicates that the encounter you selected is either shiny-locked, or cannot be the shiny type (Square, Star, or just generally wrong value) you set.
Can you give an example of the legendaries and/or mythicals you're trying to make Shiny (species, game of origin, etc)? Because several encounters in SWSH, BDSP, and PLA are shiny-locked, (as are many in past-generations).