It looks like you are keeping the soil too wet. Very gently tip the plant out of the container and get a good look at the root and potting mix. Too wet soil can cause a fungal or bacterial infection. Let the soil dry a few extra days before watering again, and go ahead and remove the infected leaf. Keep us posted.
How long has it been in that pot?
How often do you check the moisture?
How do you decide when to water?
Does water come out the bottom when you water?
Does it sit in water afterwards? How long?
How close to a light source is it?
Window or grow-light?
If window, what direction does it face?
Have you fertilized it? When and with what?
It’s probably a combination of light and water issues. The answers will help determine which.
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