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
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.
Configuring server startup options is restricted to users who can
change the related entries in the registry. This includes the
- 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.