Okay, Surendar, I've googled on ř and although all the references are in foreign languages which I don't speak, it seems likely to me that this is a reference to a font character which is not in your e-reader's font set. From your name, I might guess that it involves a character in hindi or urdu or another asian language, but on the other hand the references to rcaron and ccaron might indicate that this is a book on mathematics which involves formulae.
If this is an ebook in your control, perhaps one you have converted to ebook format yourself, you may have to embed the font file you used to create the manuscript in order for the e-reader to be able to handle it. If not, you may be able to load a suitable font onto your e-reader and use that. It depends what e-reader you have.
Just a suggestion.
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