Hi SriKoka,
yes, you can re-claim the unused licenses for your Windows Server 2019 Standard.
You can retrieve the license keys by logging into your Microsoft Volume Licensing Service Center (Volume Licensing Service Center (microsoft.com)) account. Once you have logged in, you can view your license keys.
If you do not have access to your VLSC account, you can contact Support Center Contact Info. | Microsoft Learn for assistance.
Hope it helps.
Kind regards,
Lei