Troubleshooting for "Login failed for user 'domainname\computername$'

Problem

I have received an email alert from a server:
Title: SQL Server Alert System: 'computername - Sev:014 - INSUFFICIENT PERMISSIONS' occurred on \\computername
DESCRIPTION:    Login failed for user 'domainname\computername$'. Reason: Could not find a login matching the name provided. [CLIENT: <local machine>]
The error message in the error log:

Login failed for user 'domainname\computername'. Reason: Could not find a login matching the name provided. [CLIENT: <local machine>]
Error: 18456   Severity: 14   State: 5.

Solution

My first step is to check the Event log on the server:

The actual user is Network Service. The Network Service account is a built-in account that has more access to resources and objects than members of the Users group. Services that run as the Network Service account access network resources by using the credentials of the computer account in the format domainname\computername$. The actual name of the account is NT AUTHORITY\NETWORK SERVICE.

In my case, adding computer account domainname\computername$ into the SQL Server instance solved the issue. Another option is to add the NT AUTHORITY\NETWORK SERVICE into SQL Server instance.