Thanks for that, I'll see if I can reproduce it.
So just to get everything straight, this appeared after enabling the Battlemetrics API?
Will you please verify these things:
API token scope, what permissions does your API have currently? (See plugin page for info which to set)
What player did you try to warn, does this player exist in BM (identity + steamid exists within your Server + Org in BM)?
The reason for the latter question is that the integration is dependent on the BM identity to be associated with the steam id. The plugin will do a lookup based on the steam id to make sure it is the correct player. The error can occur if the player has never been recorded by BM, or for some reason, the player is not indexed in the org yet.
I'll reach out to you in Discord DM to make communication easier for us, if you prefer that