txt to sbv conversion is probably related to the process of making new subtitles/captions for YouTube (.sbv) from a .txt text file saved in plain text format.
Your best option how to actually make new subtitles is to use one of the dedicated subtitle editor, for example the Subtitle Workshop. But nowadays some subtitles editors can be also found as web apps, just google for websites that are still working or are updated.
Updated: October 16, 2023