Plugin could not be activated because it triggered a fatal error.

Same here I opened a ticket.

Hello @kaiyang, @marbs.

We are aware of this issue and already working on the fixes. We plan to release a new version within a few hours. Sorry for the inconvenience.

Meanwhile, you can install the dev version: https://github.com/CleanTalk/wordpress-antispam/releases/download/dev-version/cleantalk-spam-protect.zip. Here is the guide: https://cleantalk.org/help/update-wordpress, Manual update.

Hello @kaiyang, @marbs.

We have fixed the issue. Please, update the Anti-Spam by CleanTalk plugin to the new 6.7 version:
https://downloads.www.ads-software.com/plugin/cleantalk-spam-protect.6.7.zip

The guide is here: https://cleantalk.org/help/update-wordpress

already perfectly solved

Thank you for the feedback @kaiyang!