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You can't upgrade automatically from Core to Professional. You need to do a manual installation of the Professional version as described in this video: https://www.akeebabackup.com/videos/63-video-tutorials/1505-abt03-upgrade-core-to-pro.html

I now understand what the problem is. You have an upload error. Yes, I know the error message says something different, but that's just a problem with the way Joomla! reports installation errors. Don't worry, it took me a while to figure out (I had to follow the code to understand what was really going on!).

The packages of our latest versions of our components are rather big. Since the default maximum upload size on most servers is just 2Mb this may cause an upload error. The best solution is to ask your host to set the following in the server's php.ini:

upload_max_filesize = 10M
post_max_size = 10M

If this is not possible, there's a good chance that the following lines in your .htaccess file may work on most servers:

php_value upload_max_filesize 10M
php_value post_max_size 10M

There is also another alternative, but it won't work on all hosts: installation by URL. Please go to the Download section of our site and select the software and version you want to download. Next to the "Download now" button you will see a link called "DirectLink". Right click on it and select "Copy link address" (the exact phrase depends on the browser, but it should have to do about copying the link or its address / URL / location).

Now go to your site's back-end, Extensions, Extensions Manager and find the Install from URL field. Note: on Joomla! 3.x you have to click on the "Install from URL" tab to see that field. Replace the contents of the "Install URL" field with the contents of your clipboard (the DirectLink URL you copied above) and click on the "Install" button next to this textbox. As long as your server supports installing extensions from URL the installation should go through.

If this still doesn't help you can always follow the manual installation instructions in our troubleshooting documentation.

I agree about the error reporting in Joomla!. The Joomla! leadership agrees with you. All of us who contribute to Joomla! do try to improve the error reporting. The Warnings page in Extensions Manager was a good start, but I bet that you don't even know it's there. IMHO it's a great idea in the wrong place. Moreover, the error message doesn't help you figure out that you should look there for more troubleshooting information. That's a Joomla! user experience failure. I am collecting these issues and put them in my kill list: whenever I have some free time / time I don't want to work on my stuff I take a peek there and start killing off those pesky bugs ;)

You're welcome! I am glad we could help you :)