Built-in accounts are used as application pool or service identities (SharePoint Server)
APPLIES TO: 
2013 2016 2019 Subscription Edition 
SharePoint in Microsoft 365
Rule Name: Built-in accounts are used as application pool or service identities.
Summary: Built-in or local computer accounts are used as an application pool identity or service identity.
Cause: Using built-in accounts as application pool identities or as service identities is not supported in a farm configuration. Built-in accounts include Network Service, Local Service, and Local System.
Resolution: Change the identity that is used for the service or application pool
Verify that the user account that is performing this procedure is a member of the Farm Administrators group.
On the SharePoint Central Administration website, click Security.
On the Security page, in the General Security section, click Configure service accounts.
On the Service Accounts page, in the Credential Management section, in the upper drop-down list, click the service or application pool for which you want to change the identity.
In the Select an account for this component list, click the domain user account that you want to associate with the service or application pool.
If you want to register the account that you selected on the SharePoint Server farm, click Register new managed account.
Click OK.