You can’t provide a value for an interval like that. You need to multiple the parameter value with your interval base unit:
"select * from orders
where created_date < clock_timestamp() - (interval '1' day) * :days"
As you are dealing with days, you can simplify that to:
"select * from orders
where created_date < clock_timestamp() - :days"
Another option is the make_interval()
function. You can pass multiple parameters for different units.
"select * from orders
where created_date < clock_timestamp() - make_interval(days => :days)"
The notation days => ...
is a named parameter for a function call. If the variable represents hours, you could use make_interval(hours => ..)