JWT invalid error when calling app store server api

Hi, This is first time posting in here.

Now our app is using In-app purchase and i'm implementing server logics using app-store-server-api.

But from yesterday, i'm facing 'http status code 401 Unauthorized error' when calling some apis.

I did many trials to fix this error and issue new key for api in app store connect. But I can't resolve this error. Even can't know what is the reason.

I attach my real code for calling 'get transaction info' api. Please give some help for me!!!!

const jwt = require('jsonwebtoken');
const fs = require('fs');
const axios = require('axios');

getApiJWTToken = () => {
        // header
        const keyId = process.env.APP_STORE_KEY_ID;
        const issuerId = process.env.APP_STORE_ISSUER_ID;
        const appBundleId = process.env.APP_BUNDLE_ID;

        const header = {
            alg: 'ES256',
            kid: keyId,
            typ: 'JWT',
        };

        // payload
        const issuedAt = Number(dayjs().unix());
        
        const payload = {
            iss: issuerId,
            iat: issuedAt,
            aud: 'appstoreconnect-v1',
            bid: appBundleId,
        };

        // sign
        const keyString = fs.readFileSync(filePath, 'utf-8');

        const token = jwt.sign(payload, keyString, {
            algorithm: 'ES256',
            header: header,
            expiresIn: '30m',
        });
        
        return btoa(token);
    };

requestSingleTransactionInfo = async ({ transactionId }) => {
        try {
            // jwt token
            const token = this.getApiJWTToken();

            const envType = process.env.APP_STORE_SERVER_API_ENV;

            const commonUrl = `/inApps/v1/transactions/${transactionId}`;
            let baseUrl;
            if (envType === 'Sandbox') {
                // sandbox
                baseUrl = 'https://api.storekit-sandbox.itunes.apple.com';
            } else if (envType === 'Production') {
                // production
                baseUrl = 'https://api.storekit.itunes.apple.com';
            }
            const url = `${baseUrl}${commonUrl}`;
            
const response = await axios.get(url, {
                headers: {
                    Authorization: `Bearer ${token}`,
                },
            });

            const { signedTransactionInfo } = response.data;

            const transactionInfo = await decodeTransaction(signedTransactionInfo);
            console.log(transactionInfo);
            return transactionInfo;
        } catch (err) {
            throw err;
        }
    };