Trying to load new sites onto existing Account using Upload Customers FBDI

Summary: We are trying to load new customer sites under existing Parties and Accounts, using Upload Customers FBDI but getting error message: "Upload not possible due to errors in related records." I was told that you cannot load sites onto manually created parties/accounts but not sure if that's true. If so, is there another way to mass upload sites onto existing parties/accounts?

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