The method getFeeRates() does not exist on Carpenstar\ByBitAPI\Core\Objects\AbstractResponse. It seems like you code against a sub-type of Carpenstar\ByBitAPI\Core\Objects\AbstractResponse such as Carpenstar\ByBitAPI\Deri...tTradingFeeRateResponse.
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation