Not a DOI
The resolution service at https://doi.org resolves DOIs. You have attempted to resolve core/books/radiofrequency-electronics/6458B85C30C64B6AE00EE2C0CB4B4851, which is not a DOI.
core/books/radiofrequency-electronics/6458B85C30C64B6AE00EE2C0CB4B4851 may be a Handle (DOIs are a special sort of Handle). If core/books/radiofrequency-electronics/6458B85C30C64B6AE00EE2C0CB4B4851 is a valid Handle you should be able to resolve it at https://hdl.handle.net.
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