SOLVED!!
I had some error in Cpanel error log.
I contacted my host server and they did...
”According to the error log the application files was not with correct permissions. I have now set 644 permissions for the files and 755 permissions for the folders”

Now I can send message to customers.
I hope it help someone has same issue.

Thank you.

I tried both, manually selected customers and Subscribers.
Both failed.
I can see error log is this,
2018-10-23 0:12:32 - database error: AbanteCart core v.1.2.13 SQL Error: Unknown column 'last_login' in 'field list'
Error No: 1054
SQL: UPDATE `customers`
SET `last_login` = NOW()
WHERE customer_id = 52 in <b>/home/ehoncana/public_html/core/database/amysqli.php</b> on line <b>109</b>

I'm not very knowledge person about code or programming, so I;m not sure this help to solve the problem.

Usually after payment on 2CO side customer should be redirected back to AbanteCart store.

Don't forget to setup INS URL here https://www.2checkout.com/va/notifications/.
Your INS URL is
http://store.com/index.php?rt=extension/2checkout/callback

Also setup Approved URL here https://www.2checkout.com/va/acct/detail_company_info and choose "Header return".
Your Approved URL is
http://store.com/index.php?rt=checkout/success

Not directly.  But you can establish a second store -  as a subdomain . 
eg.  Mainstore.com
second store  customwork.mainstore.com


The are both managed from within the same Admin panel.
And you would assign the custom work to just the second store.


Info on multi-store can be found here:
https://abantecart.atlassian.net/wiki/spaces/AD/pages/8028306/Multi-store

Order is confirmed that why customer will be forwarded to 2checkout for payment but after successful payment nothing happened on abantecart.

before that it was working fine may be after new version this happened.

kindly advice.

Regards

Adeel