You should really try to give more details when posting questions about a specific error like this one, like trigger configurations and order.
From the script names you supplied, it looks like you have two different triggers and one may have caused the other one to run (postLinkedIssue causes email to run).
You have to start with the trigger that uses email.js and check its rule. From the error message, it seems you are using a post event but the script specifically says "This script is used to send mail notification from a scheduled trigger". So change that trigger to scheduled and see what happens.