There’s at least 2 ways you can show the author box on single posts depending on whether a checkbox you have added to all single posts generated by the ACF plugin, is ticked YES as seen in the following image:
The solutions in this post, are based on this question from a member of the StudioPess community:
I am trying to setup author box on single pages. What i want to achieve is that the author box will be displayed on only those pages where i manually tick the custom field checkbox generated by ACF Plugin.
The code needed to make this work depends on whether the individual Users > Profile page setting for enabling the author box on this users posts is checked or not.
Both solutions depend on you creating a checkbox using the ACF plugin which can be ticked or unticked.
Here’s both solutions for logged in members: