This seems like it might need to be updated in the documenation.

https://nextjs.org/docs/app/building-your-application/optimizing/third-party-libraries#sending-events

Based on the link referenced in the next documentation I think all the parameters to sendGTMEvent need to be in an object and structred as such:

 
sendGTMEvent({
  event: "event",
  action: "buttonClicked",
  value: "xyz",
});

https://developers.google.com/tag-platform/tag-manager/datalayer#use_a_data_layer_with_event_handlers