We are having SharePoint 2016 Environment and deployed Nintex workflows.

Now we are trying to Mount the Nintex Content DB with Web Application but giving error.

We also checked in ULS Logs, Please find below ULS Log Error and also attached Screen shot of Power shall Management while running the command.

An error occured while trying to grant the user UserID access to the NW2010DB_AP_PRO_BizApps_Timesheet database on the SQL server, ServerName.
This may be expected if the caller does not have permission to perform this operation in sql server or if the database is read-only.
To ensure that SharePoint functions correctly, manually grant this user access to the database before performing this operation.
The user must also be granted access to the SPDataAccess group in the database.
The following information was provided with this error: Cannot alter the role 'SPDataAccess', because it does not exist or you do not have permission.