SharePoint 2019 On Prem – Modern experience – Conditional Formatting not working

Environment: SharePoint 2019 On Premise

Requirement: If a Number column value is equal to 0 then set sp-field-severity–good else set sp-field-severity–warning

Trial:
Based on the article at Use column formatting to customize SharePoint have prepared and set the below JSON

{
“$schema”: “https://developer.microsoft.com/json-schemas/sp/v1/column-formatting.schema.json”,
“elmType”: “div”,
“txtContent”: “@currentField”,
“attributes”: {
“class”: “=if(@currentField <= 0,’sp-field-severity–good’, ‘sp-field-severity–warning’)”
}
}

However it doesn’t work.

I tried to check the DOM it shows as bellow

enter image description here

Note: as per the instructions in MS article i am using https://developer.microsoft.com/json-schemas/sp/v1/column-formatting.schema.json schema as it is SP 2019 On Prem.

What am i missing?