Here are the instructions for installing utPLSQL on the Oracle Autonomous Cloud DB, running 19c (At the time of writing utPLSQL supported max Oracle v19c).
Download the package: https://github.com/utPLSQL/utPLSQL/releases/download/v3.1.10/utPLSQL.zip
Unzip it - it really does not matter where you unzip it. You can use some program like winzip to unzip it. On Mac there is a default program installed.
Open the file utPLSQL/source/install.sql in sqlDeveloper. The full path doesn't matter, on my computer it is /Users/connect/oracle/utPLSQL/source/install.sql (Mac)
Click the run script button.

When it asks write admin as the user - this is the trick. Dont try to install it as ut3 user. Just use the admin user**

** It is possible to install it as a different user (ut3) however you neeed to grant roles and permissions. I couldn't get this to work properly - I could grant the roles using the script create_synonyms_and_grants_for_public.sql but it was complaining that I have insufficient privileges. You might have to play around with user roles in the web interface https://dba.stackexchange.com/questions/290401/how-to-grant-sysdba-access-to-users-on-oracle-cloud-v19c
** You can't create tablespaces on autonomous cloud databases, just for info. There is another script that creates the ut3 user - it asks for the tablespace. Basically its just alot eaisier to install it as admin... And it works. To see it in action see the documention examples: http://utplsql.org/utPLSQL/latest/userguide/getting-started.html