Not altogether familiar with arcade, but it uses Javascript like syntax.
So your statement should look something like:
if (condition1) {
// block of code to be executed if condition1 is true
} else if (condition2) {
// block of code to be executed if condition1 is false and condition2 is true
} else {
// block of code to be executed if the condition1 is false and condition2 is false
}
For your code, something like:
if (billclass == 'D') {
eru = 0
} else if (['V','I','H'].indexOf(billclass) >= 0) {
eru = Round(parea/43560,2)
} else if (billclass == 'R') {
eru = Round(parea*(2000/7760)/2000,2)
} etc...
return eru;
Note there's no "in" operator, so even if you got the if and else if syntax correct, you'll get a Syntax Error: Binary Operator not recognised in so I've used the indexOf function and I've changed your python-like tuples ('V','I','H') to JavaScript-like arrays ['V','I','H']
I recommend you look at the arcade reference.