Parsing error in activating Pressbook plugin

I get this error with either trying to activate version 5.4.2 or 5.5.0:
Parse error: syntax error, unexpected ‘:’, expecting ‘{’ in /home/proponop/public_html/wp-content/plugins/pressbooks/vendor/johnbillion/extended-cpts/functions.php on line 59

Has anyone else experienced this. This is a new installation on a new host. The previous installation of about 4-5 weeks ago was fine, but I don’t remember which version I used).

Thanks for any help/suggestion you can offer.

A parse error like that is usually because you are not using the right version of PHP.

Pressbooks requires PHP 7+

What version of PHP are you running?

You are right. I didn’t bother checking. I’m running 5.6.37. I opened an account on a new host, and just assumed they were running 7+. Thanks!