Trying to copy one file with AWS S3 CLI

Are you confusing aws s3 sync with aws s3 cp. For copy, you need to specify the source file. The destination file can be current directory.

aws s3 cp test.txt s3://mybucket/test2.txt

Ensure that your path is correctly written. Remember add --recursive option, because is folder

aws s3 cp ./ s3://{bucket}/Test --recursive