I can’t login WordPress after plugin activated!

No panic. If you can’t login WP side after plugin activated it is a cookie setting problem.
Check (read all) this page:
https://www.axew3.com/w3/2016/02/configure-phpbb-for-cookie-on-subdomains/
follow all steps in the phpBB WordPress integration Help Install Page:
https://www.axew3.com/w3/cms-plugins-scripts/wordpress-plugins-scripts-docs/wordpress-phpbb-integration/
it contain even detailed instructions on how to disable the plugin if necessary, so do not worry.

From 1.7.2 there is an auto setting for the cookie domain, so the last part on the cookie setting page help about Reset Cookie
https://www.axew3.com/w3/2016/02/configure-phpbb-for-cookie-on-subdomains/
normally will be not necessary. Just in case, please take a look into above linked help page.

Coming 1.7.3 will be further improved about this.