For Chef Client 15.x, the 8.0.0 version of the audit cookbook is required, otherwise you will see the very confusing error shown below. Audit Mode and the audit cookbook are two totally separate mechanisms that served a similar purpose.
Audit Mode has been long obsolete, the audit cookbook does its entire job using Inspec now.
Chef Infra Client
|Chef Client||Audit Cookbook version|
|>= 15||>= 8.0.0|
Note: When used with Chef Client 15 and above, the Audit cookbook must be >= 7.7.0. Otherwise, you will see the following failure.
Recipe Compile Error in /var/chef/cache/cookbooks/audit/recipes/default.rb ================================================================================ RuntimeError ------------ Audit Mode is enabled. The audit cookbook and Audit Mode cannot be used at the same time. Please disable Audit Mode in your client configuration.