Add plugin not available

Hello All!
I’ve install WordPress 4.9.8 on my lighttpd server for my father.
It seems to work, wordpress presents itself and we can login to wp-admin, but there is no “add plugin” link.
Searching the forum, there are numerous suggestions but non seem to work for me…
I then decided to install the “Health Check & Troubleshooting” plugin by hand, and the plugin appears, but clicking on the “troubleshoot” changes the url in the address bar but further does nothing…

I’m completely stuck now…
Could someone please help me with this?

Thanks!
Geert