@DLH2 I just had this happen last night.  I found the following: 

Authentication failures are often caused by firmware updates, password complexity, or incorrect third-party compatibility settings. Users frequently encounter errors like TRANSPORT_UNKNOWN_CREDENTIALS_ERROR or "Unable to authenticate" when adding Tapo or Kasa devices, particularly after firmware updates (e.g., Tapo P110 firmware 1.4.0+ or HS300 updates) that change encryption requirements. 

Common solutions to resolve credential issues include:

• Toggle Third-Party Compatibility: In the Tapo or Kasa app, disable "Third-Party Compatibility," wait a moment, then re-enable it and restart Home Assistant. This is often required after firmware updates or if the device was added to the app after the initial setup. 

• Remove Special Characters: Change your Tapo password to remove special characters, as some firmware versions reject them during the handshake. 

• Manual IP Entry: If the device is discovered but fails authentication, try manually adding it by entering the device's IP address directly in the TP-Link Smart Home integration settings. 

• Factory Reset: For persistent issues, factory reset the smart plug, re-add it to the official app (ensuring Third-Party Compatibility is on), and then restart Home Assistant to force a fresh discovery. 

• Check Case Sensitivity: Ensure your username/email matches the Tapo app exactly, as some users found that capitalization issues (e.g., capital letters in the email) caused authentication failures. 

If the device uses Matter, adding it directly to Home Assistant may fail; in some cases (like Meross plugs), sharing credentials via Google Home first allows Home Assistant to pair successfully.  Ensure no other custom integrations (like Tapo Controller) conflict with the native TP-Link Smart Home integration. 

The fix for me was Toggling the Third-Party compatibility in the Kasa app, and I went ahead and reset my password to remove symbols.