The client ID may be missing or incorrect in the Web.config file for the app.
You could have a try with follow these steps:
Open the Web.config file for the app.
In the Web.config file, find the app key ida:ClientId.
Replace the value of the app key with the client ID that is provided for your app in the Azure AD B2C admin portal.
The changed part of the file resembles the following:
<appSettings>
<add key="ida:ClientId" value="**xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx**">
</appSettings>
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