If you entered the OTP but it says invalid, you can try the following steps:
1: Enable API access from the portal.
2: The system must be connected to the internet.
3: Make sure you are checking from registered mail or mobile
4: Make sure you are using the latest version of BUSY.
5: You can contact the GST portal helpline.
6. Also, please make sure from Administration, then select Configuration, then select Features / Options, then in the GST/VAT option, you must check the GST portal user name is correct.