Hello,
I don’t know if you already solve the problem, but I faced the same issue when I has trying to install nextcloud yesterday.
The issue is a bug in the last version of nextcloud server (17.0.2), there is a github issue here.
To solve the problem you need to install the previous version that you can find here or directly download the zip file.
Remove the nextcloud folder you have and start the tutorial where the author asks you to download the nexcloud zip, download the previous version and follow the next steps.