Essentially, there are 'permissible statuses' for managing findings within Audits.
The Permissible statuses for finding management, which are set from your Module Settings, will dictate whether or not you can close a finding within an audit.
If the Audit isn't in one of the permissible statuses you will not be able to close a Finding.
Also, if the Audit is in the Closed status you can't make any changes to the Findings.
This is the message you will likely see if you can't close an Audit Finding:
On further investigation we can see that the Permissible statuses for finding management are set to In Progress and Audit Performed.
These will need to be updated to allow you to close an Audit Finding.