Can you check that you’re not already in the post context? Without looking at the rest of the file it’s hard to tell, but from what I can see, it looks like you might be trying to go into the post.post context, which doesn’t exist
Because you’re in the primary_author context, the reading time helper won’t work - what you’re effectively saying is “Give me the post.primary_author.post and generate the reading_time from that”.
I’m not sure if this is the only way, but one fix is to close the primary_author context right before you run the helper. You also might be able to get it to work by doing {{../reading_time}} but I’m not entirely sure.