The contact specified has possibly been merged (or archived). Xero does not indicate if the contact is merged, therefore AutoEntry cannot know for sure, please contact email@example.com if this contact is not merged. You can vote for Xero to add this feature here.
This error can appear for two reasons:
You have archived the supplier/customer contact in Xero
You have merged the contact with another contact in Xero
To clear this error, you will need to do the following: