Don’t activate a “Poka Demo Import” plugin. In v4 you should activate only One Click Demo Import, then go to Appearance → Poka Demo Import to import the demo.
To remove the critical error, deactivate/delete whatever you activated called “Poka Demo Import” (Plugins → Installed Plugins). If you can’t access wp-admin, rename its folder inside wp-content/plugins/ via FTP/File Manager.
Update to the latest PokaTheme v4 (don’t use v4.0.0). Older builds can crash on newer WordPress/PHP setups.
If you need extra help, email support with your site URL, PokaTheme version, WordPress/PHP versions, and the exact error text from the WordPress email or debug.log.