Hi @anupent,
Yes, I have tried with multiple other names aside from Admin.
I should have added that I am not fluent in PHP and other languages. I understand the basics and am in the process of learning, but I am still a newbie. Though I can learn with the right instructions.
I am using MAMP 3.0 as the server, which is running PHP 7.0.
But your question prompted me to check the MAMP log, and there is a long list of error messages that I don’t understand.
E.g. the very first error is: [26-Jan-2018 12:20:23 Europe/Berlin] PHP Deprecated: Methods with the same name as their class will not be constructors in a future version of PHP; Smarty has a deprecated constructor in /Applications/MAMP/htdocs/ojs-3.1.0-1/lib/pkp/lib/vendor/smarty/smarty/libs/Smarty.class.php on line 64
Also, when I load OJS the first time for installation, above the set-up details it shows the following errors. These error messages do not display when click Install, but they are obviously still there:

Again, I am new to programming languages – but I am keen to learn, so any instructions will be appreciated. If the problem is too complex for me I will seek help from a freelancer or something, unless someone wants to help a not-for-profit sauna association launch a journal.
A diagnosis would be great if someone can see the problem.
Thanks,
Jack