Error while adding openAI provider

I encountered the same issue as well. To resolve it, I ran
composer require openai-php/client
to install the required OpenAI PHP SDK, followed by drush cr to rebuild the Drupal cache. This fixed the error, and the OpenAI provider was successfully added.

I can only assume you didn't install through composer, or else had some kind of composer error whilst installing, but I cannot recreate thios issue and the required library is listed in the composer.json file for both versions, so I'm going to close this as works as designed.