Can't change authentication mode from Windows to both SQL Server and Windows As the title suggests I'm having problems changing the authentication mode from Windows to both SQL Server and Windows mode in version 12.0.2. I have tried right right-clicking the server, go to security and change it there but I'm getting an error when doing this. I installed it originally using both but changed it to windows only mode earlier this evening. 

The error message I'm getting is long winded but the end of it is:


  The EXECUTE permission was denied on the object 'xp_instance_regwrite', database 'mssqlsystemresource', schema 'sys'. (Microsoft SQL Server, Error; 229)


Any idea what that means? I'm guessing I don't have the required permission but I'm not familiar enough with it to know what to do next. I have the sa password but can't enter it while I'm stuck in Windows mode.

I've been trying to figure out why I can't seem to write to the database all day and in desperation tried changing the mode to see if that would fix it... but it's only made things worse.

While connected using Windows credentials I tried ALTER LOGIN sa ENABLE also and was given the following error message in response:


  Cannot alter the login 'sa', because it does not exist or you do not have permission. 


I did try altering the registry too to change the mode but realised I was reading a post relating to SQL Server 2005

Thanks in advance for your help.
The EXECUTE permission was denied on the object
  'xp_instance_regwrite', database 'mssqlsystemresource', schema 'sys'.
  (Microsoft SQL Server, Error; 229)


You can fix the problem by granting the user direct access to the procedure.

USE master
GO

GRANT EXECUTE ON [sys].[xp_instance_regread] TO [DOMAIN\USER];

--To veriy that the user has been granted the privilege
EXECUTE AS USER = 'DOMAIN\USER';Select * from fn_my_permissions('xp_instance_regread','Object')



  Cannot alter the login 'sa', because it does not exist or you do not
  have permission.


Error message saying that, You do not have administrator's permissions

Log into SQL Server Management Studio as an Administrator, because only admin user has permission to change passwords for users.

How to change sa user password see here: How to change a password for the sa user in MS SQL? and this link