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If you check the Private certificate requirements, then you will find that to upload or import a private certificate to App Service, your certificate must meet the following requirements:
• Exported as a password-protected PFX file
• Contains private key at least 2048 bits long
• Contains all intermediate certificates in the certificate chain
The error you are getting can also be due to Insufficient rights. You should be having admin rights on subscription to to upload pfx cert. After granting the correct rights the problem can be resolved.
You can also you key vault to import the certificate. After authorizing App Service to read from the vault, you can import a certificate from your vault to your app. Check Import a certificate from Key Vault for more information.
Check this similar problem raised in the Microsoft Q&A. If you are still facing the problem after all these I would suggest to raise a support ticket to Azure.