Check that your swat4.ini file (in Documents/SWAT4 ) is not set to "Read Only." A locked INI file can also trigger error 20 because the game cannot write its authentication cache.
However, here is the catch: using a widescreen fix often modifies the game's executable file ( Swat4.exe ). Swat 4 No Disc Inserted 20
You can find these at community sites like GameCopyWorld . Installation: Check that your swat4