How can I cancel a posted invoice? The error message 'BSEG-GSBER is not an input field' is appearing. Even OSS note -94932 did not help.

The error is due to suppression of Business area field in the field status group. Due to this, while reversing the document, the system finds this field is suppressed and hence does not proceed further.
Locate the field status group for the recco account entered in the vendor master and the relevant field status variant.
In IMG, go to FI Global settings-Documents- Line items-Control-Maintain field status variant.
Here for the relevant field status variant and the field status group you may find the business area field has been suppressed.