Account acc = new Account(Name='Test Name');
if (acc.Do_Not_Contact__pc == false) {
    System.debug('DNC is false');
} else {
    System.debug('DNC is true');
}
insert acc;
acc.Do_Not_Contact__pc = true;
update acc;

System.DmlException: Update failed. First exception on row 0 with id 001W000000fFiVbIAK; first error: INVALID_FIELD_FOR_INSERT_UPDATE, Account: bad field names on insert/update call: Do_Not_Contact__pc: [Do_Not_Contact__pc]

acc.Do_Not_Contact__pc = 'true';

acc.Do_Not_Contact__pc = true;

acc.Do_Not_Contact__c = true;