This effect is normally caused by an unsupported graphics driver.
You should check if you can get an updated graphics driver for your computer. If you can't, there is the option of adding a software OpenGL driver (MesaGL) to Aegisub's installation folder, that usually works but does make video very slow.
Do not PM me asking for help with Aegisub, Automation, karaoke or anything like that, I won't reply.
Ask your questions in the public forums. Use the Search function, somebody probably asked the same question before.
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Experimenting is free and is a great way to learn things.