access denied error on `getPresignedPost` method on AWS s3 - amazon-web-services

Question

In my REST API, I call a createPresignedPost method which gives back me as usual an url and other fields. As the returned url wasn't working, I modified it by http://${url.split('/')[3]}.s3.amazonaws.com this. After appending my file, I made post request with the url and everything worked fine. But, I opened it now after a month and now it isn't working.
I searched a lot to resolve this issue, created a new ACCESS KEY ID and ACCESS KEY SECRET, modified policies, tried with new buckets. Also, I am not able to get any simplified method of debugging my errors in amazon s3. Can anyone elaborate what exactly is the problem here. This Acccess denied message is too vague to understand.
The XML errror message is:
<Error>
<Code>AccessDenied</Code>
<Message>Access Denied</Message>
<RequestId>SYBG5Q84R219PN35</RequestId>
<HostId>CVNuh2YSb6gYqYRWPjlONc7hi8wLKfhCQC09nsmwWJz9oqUJ3mF1Ji5U39PO4Tm0xqrGxb7ql1w=</HostId>
</Error>
frontend code
const res = await Axios.get(`RESTAPI`);
const { url, fields } = res.data;
const newUrl = `http://${url.split('/')[3]}.s3.amazonaws.com`;
const formData = new FormData();
const formArray = Object.entries({ ...fields, file: blob });
formArray.forEach(([key, value]) => formData.append(key, value as any));
// Uploading file to AWS
await Axios({
method: 'POST',
url: newUrl,
headers: {
'Content-Type': 'multipart/form-data',},
data: formData,
responseType: 'text',
});
backend code
aws.config.update({
accessKeyId: process.env.AWS_ACCESS_KEY_VERCEL,
secretAccessKey: process.env.AWS_SECRET_KEY_VERCEL,
region: process.env.AWS_REGION_VERCEL,
signatureVersion: 'v4',
});
const s3 = new aws.S3();
const fileName = `fileName`;
const params = {
Bucket: process.env.AWS_BUCKET_NAME,
Key: fileName,
Expires: 10000000, // seconds
Conditions: [
['content-length-range', 0, 1048576], // up to 1 MB
],
};
await s3.createPresignedPost(params, (err, data) => {
if (data) {
return res.status(200).json(data);
}
if (err) {
return res.status(500).json({ message: 'Upload Failed!', type: 'subtitle' });
}
});