Einstein bots activate button is greyed out

If the bot is created first time after the feature was recently enabled, then try toggling the feature off and back on. Wait for few seconds to ensure that it was turned off properly. If other bots are working in the org then most likely the issue is specific to bot configuration such as a custom entity is not setup correctly.

To look for error details, hover over the activate button and it will show the message. Fix the configuration as per message displayed otherwise contact Salesforce support for further guidance.