The MailChimp PHP library is missing the required GuzzleHttp library.

I am having the same issue. Did you solve it?

Are you using Composer Manager? You cannot use Composer in Drupal 7 without 1)Composer Manager module 2) custom module to load composer files 3) hack core to load composer files.

No problem!

For anyone not wanting/able to use Composer:

Download:
https://github.com/thinkshout/mailchimp-api-php/releases/download/v1.0.5...

* Replace sites/all/libraries/mailchimp with the downloaded (unzipped) content.
* Rename v1.0.5-package to mailchimp
* Make sure the file-permissions are set correctly

This mailchimp library includes the guzzlehttp library.

-- source: README.txt from the new MailChimp module.

#8 works for me Thank you Fonteijne

Had the same issue, #8 fixes it for me. Though, the real issue for me I believe was .gitignore was excluding the vendor folder.

Thanks Fonteijne! #8 worked like a charm!

Thanks KRX ! you save my day !

Thanks #10! for me was the .gitignore.

Just ran into this issue on Pantheon.io. Composer manager is the fix.

So running the composer install command in sites/all/libraries/mailchimp as suggested in the README was not the way to go in this case because the vendor directory created by Composer was not committed (silently) to Dev and this generated errors like

The MailChimp PHP library is missing the required GuzzleHttp library.

So if your site is hosted on Pantheon, using composer_manager will consolidate all vendor directories @ sites/all/vendor which means all the downloaded requirements will properly be committed to Dev, Test and Live environments.

You can also use git add -f to add the vendor folder to your repository.

Thanks @Greg Boggs. Can you expand? Why is this required for vendor?
Knowing is good. Understanding is better :-)

This is standard composer practice. Vendor folders are in .gitignore so that you don't accidentally commit them to your git repository. However, in the case of Pantheon, you're using git as a deployment tool, and Pantheon does not support running composer. So, you either need to use an external tool like Circle CI to run composer install on deploy. Or, you just need to force add your dependencies for Pantheon because you are in a special case where you DO want your vendor dependencies. But, other folks not on special hosting don't do it this way because they would add many hundreds of megabytes of files to the repository.

Now I Understand, thanks. Interesting.

So one way or another, when hosting on Pantheon and not using Circle CI or similar tool, one has no choice but to add the vendor directory to the repo. Is that it?

Yep. Even if you use Circle, Circle adds the vendor folder to the repo. Most folks dont' want it because they use Git to store their source code. But, Pantheon uses git to store the production website. So, it's a special case.

I found a solution that might help people https://www.drupal.org/project/mailchimp/issues/2054037#comment-12789377