You are missing a couple of curly brackets. Each return statement needs to be enclosed by curly brackets. Also, you need to use a double equals sign when checking if something is equal to a value. A single equals sign is used for assigning variables.
if($feature.MAJOR_ST == 'Farm'){
return "No"
} else {
return "Yes"
}
Refer to If - else | ArcGIS Arcade | ArcGIS Developers for info on if-else syntax.
Refer to Operators | ArcGIS Arcade | ArcGIS Developers for operators, which discusses = vs ==.
For straight forward if-else statements, arcade also has an IIF function, which can be a little easier to create:
IIF($feature.MAJOR_ST == "Farm", "No", "Yes")
Logical functions | ArcGIS Arcade | ArcGIS Developers