Please improve the warning by specifying what character specifically could not be found.
The character used for Underline is not available in font asset [merriweather_black].
UnityEngine.Debug:LogWarning(Object, Object)
TMPro.TMP_Text:GetUnderlineSpecialCharacter(TMP_FontAsset) (at Library/PackageCache/com.unity.textmeshpro@2.1.6/Scripts/Runtime/TMP_Text.cs:6081)
TMPro.TMP_Text:GetSpecialCharacters(TMP_FontAsset) (at Library/PackageCache/com.unity.textmeshpro@2.1.6/Scripts/Runtime/TMP_Text.cs:6007)
TMPro.TextMeshProUGUI:LoadFontAsset() (at Library/PackageCache/com.unity.textmeshpro@2.1.6/Scripts/Runtime/TMPro_UGUI_Private.cs:574)
TMPro.TMP_Text:set_font(TMP_FontAsset) (at Library/PackageCache/com.unity.textmeshpro@2.1.6/Scripts/Runtime/TMP_Text.cs:174)