I have no idea why there is an error when converting the subtitles. Please post the log here. (I can't test the problem myself because I don't have the DVD.)
A stream can be globally marked as forced in the VTSI_MAT table (like all other information about the subtitle: for children, director comments, large, normal etc...) But that info is usually not specified (another thing that most authors of commercial DVDs do very badly), and the fact that a stream is not marked as globally forced doesn't mean that it is not forced. Furthermore, a subtitle track can have some forced subtitles and other subtitles that are not forced. For that reason, the SetSTN command has a parameter to display all subtitles or only the forced ones.
In the subtitle stream, each individual subtitle has a "forced" flag that can be set or clear. The IFO contain the "forced subtitles" information only for the whole stream, but cannot tell that a stream contains only SOME forced subtitles. When the stream is converted by BDSup2Sub, it can extract only the subtitles with the forced flag set and generate an output stream with the forced subtitles only. That's handy, since most (all?) MKV players cannot distinguish the forced subtitles in a stream with forced and unforced subtitles. So, if you want to let the user chose to display either all subtitles or only the forced subtitles with any player, you have to generate two streams: one with all subtitles and the other with the forced subtitles only.
Unfortunately, since it is not possible to know if a stream contains some forced subtitles just by analyzing the IFO, it is only possible to know that after the demux of the whole stream. So, the plugin presents you the two streams in its GUI: the original full stream, and a "pseudo-stream" with the forced subtitles only. If you select the latter one, BDSup2Sub will be used to extract the forced subtitles and generate a "forced only" stream. Of course, if there are no forced subtitles at all in the stream, it will not be possible to generate that stream, and PGC2MKV will simply issue a warning in the log and skip the forced only stream.
Note that SubtitleCreator can be used instead of BDSup2Sub to convert the subtitles, but SubtitleCreator cannot extract the forced subtitles only. Therefore, if you have selected SubtitleCreator in the settings of the plugin, you will not see the "FORCED ONLY" pseudo-streams in the GUI. Also, the plugin will not add the forced only pseudo-stream in its GUI if the stream has already a global type like Director Comments, as it doesn't make sense to flag some subtitles as forced in that kind of stream. And of course, if the whole subtitle has the forced flag in VTSI_MAT, the forced only pseudo stream is not added, since by definition all subtitles are forced (even if they do not have the individual forced flag set).
[NOTE to other readers] This reply and the previous post by hubblec4 follow a
discussion started in another thread. A new beta version of PgcEdit 9.4 and the PGC2MKV plugin, not officially released yet, are necessary.