[HELP] after encoding ass into mp4, but there is no subtitle at all

[Selur]

The name must be the same as the font name.
I checked the font name using IrfanView, but you can also copy the ttf into your Windows\Fonts folder, den double click on the font to see the font name. The font name is what needs to be used.
No clue what Aegisub does, but at least here '方正悠黑简体 508R' isn't the name of the font, but it might work with that name on your system.

Cu Selur