Description and Resolution of System.SObjectException: Field is not writeable

Was working on the apex, and found this error, its a System exception, so you cannot handle it easily until you resolved it.

So basically this error comes when you are trying to re-assign the parent record on child object, in a master detail relationship.

For the resolution you can do "Allow  reparenting" is one of the resolution by which in the apex you can re-assign the parent field.

You can go to the child object and on the master-detail field, and check Allow reparenting.

Thank you.!!