System.TypeException: DML operation INSERT not allowed on __Share - salesforce

Question

I have created share object in my salesforce environment.
From Setup-> security Controls -> Sharing Settings -> I made my custom object access to "Private". So suppose my object name is EVT_Client__c & I am trying to insert data into EVT_Client__Share from trigger(AfterInsert) I am unable to do it. I am getting error as given below.
Once data get inserted into EVT_Client__c object our trigger tries to insert data into EVT_Client__share object but we are getting error. Profile user who is inserting data into EVT_Client__c object through UI having Read/create/edit/delete access on object.
System.TypeException: DML operation INSERT not allowed on
EVT_Client__Share
My code in trigger is as below in which I am getting error.
public with sharing class EVT_Client_TriggerHandler {
private void ShareClientToThirdPartyJobGroup(List<EVT_Client__c> lstClients){
List<EVT_Client__Share> lstSharesForThirdPartyGroup = new List<EVT_Client__Share>();
List<Group> lstThirdPartyGroups = [Select Id, RelatedId from Group where Name = 'Third Party'];
for(EVT_Client__c client: lstClients){
for(Group roleGroup : lstThirdPartyGroups ){
EVT_Client___Share objShareForThirdPartyGroup = new EVT_Client__Share(ParentId = client.Id,
UserOrGroupId = roleGroup.Id,
AccessLevel = 'Edit',
RowCause = Schema.EVT_Client__Share.RowCause.User_Client__Access__c);
lstSharesForThirdPartyGroup.add(objShareForThirdPartyGroup);
}
}
insert lstSharesForThirdPartyGroup;
}
}

Answer 1

You're sure it's not a "detail" in any master-detail relationship? The OWD would then say "controlled by parent" instead of "private". Can you switch to Classic view, go to page layout and verify the [Sharing] button can be added, displays OK, this user can manually share this record from UI?
Post your code sample? I suspect you need to define Apex Sharing Reason first. It's bit like roles if you ever played with Account/Contact/Opportunity/Case roles.
Once it's set you should be able to do something like this code sample: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_bulk_sharing_creating_with_apex.htm (they use "manual" as sharing reason but you might have more luck with custom sharing reason.
I quickly created your object and this worked like a charm for me:
trigger Stack57201752 on EVT_Client__c (after insert) {
List<Group> groups = [SELECT Id FROM Group WHERE Name = 'Third Party'];
List<EVT_Client__Share> shares = new List<EVT_Client__Share>();
for(EVT_Client__c c : trigger.new){
for(Group g : groups){
shares.add(new EVT_Client__Share(
ParentId = c.Id,
UserOrGroupId = g.Id,
AccessLevel = 'Edit',
RowCause = 'Manual'
));
}
}
insert shares;
}
And when I click the [Sharing] button on the record it looks good:
So... Stupid question time: have you clicked this checkbox?