Make a field mandatory based on another field

[Alice]

Hello need help please
I have to make the system field due date mandatory only if the custom field delivery date desired is filled. Here is my script:

import com.atlassian.jira.component.ComponentAccessor
import com.onresolve.jira.groovy.user.FormField
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.issue.CustomFieldManager

def issueManager = ComponentAccessor.getIssueManager()
def customFieldManager = ComponentAccessor.getCustomFieldManager()

def desired_delivery_date = customFieldManager.getCustomFieldObject("customfield_12303")
def issueObject = issue
def duedate = issueObject.getDueDate()

if ( desired_delivery_date != null){
      duedate.setRequired(true)
}

When I execute this script I get this error : 

groovy.lang.MissingMethodException: No signature of method: com.atlassian.jira.issue.fields.ImmutableCustomField.setRequired() is applicable for argument types: (Boolean) values: [false] at Script1288.run(Script1288.groovy:21)

Thanks for your help