Please remember that extension installation and updates are handled by Joomla itself, not our code. What you are experiencing is a known bug in Joomla 3.9.0. It is already fixed and the fix will be included in Joomla 3.9.1.
In the meantime you can download the latest version of Admin Tools from our site and use Upload & Install to install it in Joomla! without uninstalling the previous version. This has exactly the same effect as installing an update (under the hood, when installing an update, Joomla downloads the latest version’s ZIP file and installs it.
I hope this information helps.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!