Oracle Enterprise Manager Database Express, or EM Express, is a web-based tool used to manage Oracle 12c databases. In order to ensure that the user connecting to the application can only see the information and do not make any changes, the relevant database user must be granted the role EM_EXPRESS_BASIC.
You can Grant EM_EXPRESS_BASIC role to the relevant database user as follows.
SQL> grant EM_EXPRESS_BASIC to <USER>;
In this way, when the user is logged into EM Express application, he/she will only be able to see, and can not make any changes.