Hi,
Your license key is locked to a single domain, so please make sure the domain name licensed with the key matches the domain your are using. You can edit the domain any time on your profile page.
If you are developing on localhost, the watermark will not be removed but there will be a log saying:
PDFJS.express: Development environment detected. This license key is currently registered to YOUR_DOMAIN
Please refer to the documentation.
Thanks,
Logan