I am so sorry to hear you’re having trouble here. This error would suggest the Jira API rejected the payload send by n8n. Unfortunately I haven’t really used Jira for quite a while now outside of a few tests so don’t have any great guess as to what exactly might be wrong here.
Is the Jira node working fine for you when running on its own using static data (rather than data coming from a Google sheet through expressions)?
Hello @MutedJam thanks for your reply! I just discovered that it was due to my date formating. The board I was using had yyyy-dd-mm format so I had to format my data but it worked!