react-native digital signature with rsa-sha256

Help me! I have been trying to fix this for 2 days but it was completely hopeless

signer = crypto.createSign('RSA-SHA256');
try {
    signer = signer.update(value);
    console.log(signer);
    signa = signer.sign(private_key, 'base64')
    console.log(signa)
} catch (e) {
    console.log(e)
}
signer.end();


console.log('Digital Signature: ', signa);

return signa

}

I'm use react-native-crypto. This is what I got