Hello amypolly,
apologize for the inconvenience you are experiencing.
As you mentioned, Windows 10 LTSC evaluation is valid for 90 days. If you see an expired license key message immediately after installation, it may be due to the fact that you are using an expired evaluation ISO file or that your system is not timed correctly. So, please make sure you are using the latest version of the evaluation ISO file and check that your system time is correct.
Here is the link: Windows 10 Enterprise | Microsoft Evaluation Center.
Best regards,
Lei