Hi Ton
It sounds like you are trying to load a Font Awesome 5 icon. Loading FA5 will most likely resolve the issue however this can negatively effect any FA4 icons on your website.
To update to FA5...
1. Disable
Font Awesome in the
General -> Typography tab of your template settings.
2. Add the following to the
Custom CSS -> Custom CSS field of your template settings...
[class^="fa-"]::before, [class*=" fa-"]::before {
font-family: "Font Awesome 5 Free";
font-weight: 900;
}
3. Add the following to the
Custom Code -> Head Custom Code field...
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.15.1/css/all.css">
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.15.1/css/v4-shims.css">
Ciaran