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.
Here is a sample module that does the required task.
https://github.com/stpaultim/field_change_log
I'm still working on it and testing it. But, initial tests are good. ...
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.