new

The code you've posted looks OK as far as I can tell.  Just for troubleshooting purposes, you should comment out the a!save() statement that interfaces with "ri!Vehicle" and see if the same sort of error persists.  Any subsequent troubleshooting suggestions would hinge on how this behaves.

As a side note, your example here is short enough to just paste as text without issues, but if you'd use a Code Box instead you'd be able to post the code with indentation preserved, which makes it a lot more readable.

a!dateField(
  label: "Date of Manufacturing :",
  required: true(),
  value: local!manDate,
  saveInto: {
    local!manDate,
    /*a!save(ri!Vehicle.ManufacturingDate,save!value)*/
  },
  helpTooltip: "It will be present on the Registartion details",
  validations: if(
    local!manDate >= today(),
    "Manufacuted date should not be Today or Future",
    {}
  )
)