That is because you are working as admin and role of admin is limited to some features when it comes to reporting. I recommend you to create a new user account, and provide role of "Pro Author" in reporting role. With this role you can create custom reports in query studio as well as report studio.
But remember, because of limited # of users license, only one user is allowed to be part of "Pro author" role.
Each user account is assigned a role with a set of permissions. The number of users is limited by the
number of Reporting licenses attached to each role. Each role can perform the following actions:
Recipient: Views OnCommand Insight Reporting portal dashboards and reports and sets personal preferences such as those for languages and time zones. Note: Recipients cannot create reports, run reports, schedule reports, export reports, nor perform administrative tasks.
Business Consumer: Runs reports in Business Insight (in standard mode) and runs reports interactively in addition to performing all Recipient options.
Business Author: Views scheduled reports, runs reports interactively, and creates some reports in addition to performing all Business Consumer options.
Pro Author: Creates reports in addition to performing all Business Author options and has advanced reporting tool options that are available also with Report Studio.
Administrator: Performs reporting administrative tasks such as the import and export of report definitions, configuration of reports, configuration of data sources, and the shutdown and restart of reporting tasks.
There is a nice table showing which features are available per user role in this guide as well.
Please note that you need to have the "Insight Plan" license to be able to run Query Studio. That is what is meant by (OCI Plan). Check your licensing in OCI License Portal. Without the Plan license, you will not be able to select ProAuthor (and even if selected by default for Admin, the Query Studio is not available)