A user has reported an error when trying to merge two contacts using NPSP Contact...

(https://trailhead.salesforce.com/en/content/learn/modules/contact-and-account-settings-in-nonprofit-success-pack/merge-duplicate-contacts-and-accounts-npsp)

When encountering errors while trying to merge contacts using the NPSP Contact Merge tab, the following issues are likely causes:

Delete Access to Contacts:

The user needs to have delete access to the contacts being merged. Without the necessary permissions, the user will not be able to complete the merge operation.

Ensure that the user's profile or permission set includes delete permissions on the Contact object.

Unique External ID:

If a unique external ID is retained from the non-master contact, it can cause conflicts during the merge process. The system expects unique external IDs to remain unique, and retaining an ID from a record that is being merged (and effectively deleted) can create issues.