I have been working with this for a couple of days and I can't get past this same error.
The template validation failed: 'The action(s) ''Get_Attachment_content' refferenced by 'inputs' in action "Append_to_array_variable_2' are not defined in the template.
(I'd upload a screenshot, but it isn't allowing me to upload it here for some reason.)
I think part of the reason is that some changes to Flow tempaltes have been made since this was written, but I can't figure out what to change to correct this error. It's just outside of my Flows knowledge, so I was just trying to replicate what was created in this SOLUTION.
EDIT: I FINALLY got it to work. Dummy me, I needed to add _2 to part of the name. BUT - Now the issue is that it doesn't attach the uploaded attachment to the initial email. It only attaches to the approved/rejected email. Reason here is because there is no template for Send Approval Email for Outlook in Flows. You have to start an approval with a condition attached to it of "if yes" and "if no". I can't get it to work with an approval email like what is shown above.
Any chance anyone has some ideas?
Thanks!
Kelli