Startup Parameters greyed out in SSCM for new instance
As stated in the comment, please check your privilege. This change you are trying to make will write startup parameters to the registry and need privilege beyond member of sysadmin role in SQL Server.

Ref: SCM Services - Configure Server Startup Options

Limitations and Restrictions


  SQL Server Configuration Manager writes startup parameters to the
  registry. They take effect upon the next startup of the Database
  Engine.
  
  On a cluster, changes must be made on the active server when SQL
  Server is online, and will take effect when the Database Engine is
  restarted. The registry update of the startup options on the other
  node will occur upon the next failover.


Security

Permissions

Configuring server startup options 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.