Hi leah Graves,
Welcome to Microsoft Community.
Valid account credentials (such as password matching the local account) are a prerequisite for using advanced options (for example, Command Prompt) in Windows Recovery Environment.
We may not be able to use the new Microsoft account to modify the old credentials stored on the device, the password/Microsoft account password corresponding to the local account/Microsoft account is not stored directly in plaintext on the device and may need to be encrypted and converted. The new Microsoft account password for the new Microsoft account does not directly have a direct impact on the original credentials.
If the credentials are unavailable, one possibility is an anomaly specific to the account Profile that is not easily traceable.
If there is no other alternate local account and resetting the computer prompts for a password but the password is invalid, it means that a clean install of Windows almost needs to be considered.
In addition to clean installation, content in the following threads mentioned unconventional ideas summarized by other community members.
If I misunderstand your situation, feel free to correct me and share the information.
Best Regards,
Kyo - MSFT | Microsoft Community Technical Support