Greetings,
I have the same problem but in version 3.1.2 and the solutions here are not working. I am running OJS 3.1.2 updated from 3.1.1.4 updated from 2.4.8 I have PHP 7.2.14 and MySQL Database 5.6.43 The plugin in present in my ftp and the reports plugins are too only the usage statistics plugin is missing from my plugin list thus I am not able to activate my stats for my readers to see.
Please help solve this issue!