It looks like that field must be of type "number" and be limited to one value like you see in the image above. The "number of values" fieldset should be on the content type's manage fields page, e.g. "admin/structure/types/manage/<content-type>/fields/<field-of-type-mailchimp-subscription>".
I would check the manage field page to make sure it is set to one for the number of values allowed.
Worth mentioning as it was in zulip chat by @yorkshirepudding
I just tested and the two work together without issue as long as you don't use the "Exclude comments from Content...
There is also Node Comments Search - this searches the node and all comments (default search is node and first page of comments) as part of the content search. Izzy's module above is a separate...
Posted3 days 23 hours ago by Martin Price | System Horizons Ltd (yorkshirepudding) on:
If there is no response, alternatively, you can use a View with a list of comments, with a filter based on the comment text. I haven't tried it myself, I just got an idea.
Comments
The specific error you are seeing comes from https://github.com/backdrop-contrib/mailchimp/blob/1b970f66c112e88d2ccfe....
It looks like that field must be of type "number" and be limited to one value like you see in the image above. The "number of values" fieldset should be on the content type's manage fields page, e.g. "admin/structure/types/manage/<content-type>/fields/<field-of-type-mailchimp-subscription>".
I would check the manage field page to make sure it is set to one for the number of values allowed.