By default, the OK Code field is not visible in SAP Belize theme - meaing that you cannot enter a transaction code on the screen.
In order to display the OK Code field in the SAP Belize theme, you have to navigate through the toolbar menu and click on "Show OK Code Field" as per below path:
More > GUI Actions and Settings > Settings
A few things to remember:
This is a one time configuration (as long as you don't clear your browser cache) and has to be done individually for each user.
If you clear your browser cache, the OK Code field will be hidden again.
The OK code field is NOT available in the SAP S/4HANA Cloud Edition.