An error occurred subscribing mytestemail@gmail.com to list XXXXXXXXXX. "400: Invalid Resource - Your merge fields were invalid. a:1:{i:0;O:8:"stdClass":2:{s:5:"field";s:7:"MMERGE5";s:7:"message";s:20:"Please enter a value";}}

I was receiving this error when submitting a webform with a mailchimp handler:

An error occurred subscribing mytestemail@gmail.com to list XXXXXXXXXX. "400: Invalid Resource - Your merge fields were invalid. a:1:{i:0;O:8:"stdClass":2:{s:5:"field";s:7:"MMERGE5";s:7:"message";s:20:"Please enter a value";}}"

I have a field with key "fname" on the form, and I have this text added to the MERGVARS in the Mailchimp handler:

FNAME: [webform_submission:values:fname]

After much searching, I realized that "MMERGE5" referenced in the error message was referring to a merge field in that audience's settings (https://us18.admin.mailchimp.com/audience/merge-fields/), which was marked as required for some reason. Once I turned off "required" for that merge field, the issue was resolved.

Hopefully this will help others (and probably future-me as well) who may run into this issue.