How to Start Express SQL Server instance in Single user mode?
According to the documentation, SQL Server Configuration Manager requires Local Administrator permissions since various SQL Server Configuration items are stored in the local system registry.

From the "SQL Server Properties (Startup Parameters Tab)" in the SQL Server Configuration Manager Help:


  Use of this page is restricted to users who can change the related entries in the registry. This includes the following users.
  
  
  Members of the local administrators group.
  The domain account that is used by SQL Server, if the Database Engine is configured to run under a domain account.
  


You can confirm membership in the local Administrators via one of the following methods:


Start a command-prompt on the machine where SQL Server is installed, and run this:

whoami /groups | find /i "BUILTIN\Administrators"

Assuming the machine is a member of your Active Directory Domain, you can run the "Active Directory Users and Computers" Microsoft Management Console, either from the Domain Controller for your domain, or from your workstation if you install the tool by downloading the Microsoft Remote Server Administration Tools.  Once you have "Active Directory Users and Computers" running, you need to find your user account, then check the "Member of" tab to see if you are a "Domain Administrator" or "Enterprise Administrator".
On the server where SQL Server is installed, you can check the "User Accounts" from "Control Panel".