Every time I open a page to write Amharic language using SIL Ethiopic font, it jumps back to the English font instead. This happened since I installed Catalina system software (MAC). Please help, thanks.
In what applications do you experience this?
This sounds more likely to be a keyboard/input method issue than a font issue.
Although, I would recommend that you download and install the latest version of the SIL Ethiopic font which is called: Abyssinica
Are you sure that you have an Amharic/Ethiopic keyboard reinstalled correctly and active in Catalina? If it gets deactivated you will have to reselect it so that it is active in the text field of the application you wish to use.
Are you using a native Mac keyboard layout, or a Keyman keyboard, or some other input method/software?
BTW, the SIL font for Ethiopic also contains the basic set of Latin characters, plenty to for all the needs of the English language. That is why you need an Ethiopic/Amharic keyboard to access the ፊደላት.