User Interface for Genshin Cutscenes Demuxer

[Game directory]\Genshin Impact game\GenshinImpact_Data\StreamingAssets\VideoAssets\StandaloneWindows64

Try to update your GI-cutscenes script to the latest version.
Add the GI-cutscenes.exe file next to the exe file you are running.

Try to update your ffmpeg to the latest version.
Add the ffmpeg.exe file next to the exe file you are running.

You must enable merging. Then you need to select a subtitle provider.
This can be either one of the already suggested options.
But you can also download subtitles to your computer and specify the path to the folder with subtitles.
The path to the subtitles should look like this: [Subtitles folder]\LANG\Cutscene_name_LANG.srt

Install the font that is located along the path:
[Game directory]\Genshin Impact game\GenshinImpact_Data\StreamingAssets\MiHoYoSDKRes\HttpServerResources\font