The action that causes an error when creating a MySQL DB system is using the username "mysql.sys". This is because "mysql.sys" is a reserved username for the MySQL system schema, which contains views and stored procedures that implement server functionality1. You cannot create a user with the same name as the system schema, as it would cause conflicts and errors1. You should choose a different username for the administrator of the MySQL DB system, such as "mysql.admin" or "mysql.user".
The other actions do not cause errors, but they may have some implications:
* Storage size of 50 GB is not too small, but it may not be enough for your database workload and growth. You can choose a storage size from 50 GB to 16 TB when creating a MySQL DB system2. You can also scale up the storage size later if needed2.
* Host name of "mysql-host" is valid, but it should be unique within the subnet where the DB system is deployed2. You can use letters, numbers, and hyphens in the host name, but it must start and end with a letter or a number2.
* Maintenance Window start day and time is not required, but it is recommended to specify a preferred time for applying maintenance updates to the DB system2. If you do not specify a Maintenance Window, Oracle will assign a random day and time for the maintenance updates2.
References:
* MySQL :: MySQL 8.0 Reference Manual :: 5.6 The mysql System Schema
* MySQL :: MySQL Database Service User Guide :: 3.1 Creating a MySQL DB System