azure B2C inheritence errors

Question

I created a policy in Azure B2C, downloaded it and made some customizations. When I try to upload it I get these inheritance errors that I do not understand. I get the same error even if I don't make any changes and just try to upload the policy that I just downloaded. What is this error in reference to? Why is just not overwriting the policy with the uploaded policy?

Unable to upload policy. Reason : Validation failed: 1 validation error(s) found in policy "B2C_1A_B2C_1_SIGNIN" of tenant "m60b2c.onmicrosoft.com".Policy 'B2C_1A_B2C_1_Signin' of tenant 'm60b2c.onmicrosoft.com' is not allowed to inherit from the specified base policy. Inheritance chain: {
 "TenantId": "m60b2c.onmicrosoft.com",
 "PolicyId": "B2C_1_Signin",
 "TenantObjectId": "fb3c2543-8e76-443a-bf5d-4f14f78dbb9b",
 "Root": true,
 "Derived": {
 "TenantId": "m60b2c.onmicrosoft.com",
 "PolicyId": "B2C_1A_B2C_1_Signin",
 "TenantObjectId": "fb3c2543-8e76-443a-bf5d-4f14f78dbb9b",
 "Rule": "All",
 "InheritanceAllowed": false,
 "Reason": "Policy 'B2C_1A_B2C_1_Signin' in tenant 'fb3c2543-8e76-443a-bf5d-4f14f78dbb9b' is blocked from inheriting policies from 'b6291105-4814-4453-831c-7aba93c480b2' as the basic policy constraint handler 'B2CBasicPoliciesOnly' cannot match the policy id to a prefix or registered policy id."
 }
 }

Answer 1

You can only edit a built-in policy using the Azure AD B2C portal.

Downloading, modifying and then uploading a built-in policy isn't supported.

If you are wanting to customize the sign-in flow, then you must use a custom policy.