If you are not receiving the OTP while Downloading Gstr return in BUSY you can check these things:
Step 1:First you can go and check in Administration,click configurtion,click features and options,click GST/VAT tab,Here you can check GST PORTAL user name should be correctly mentioned,
Step 2:If its correctly mentioned check there should be no space in portal user name.
Step 3:API Access should be given from GST PORTAL,
Step 4:To give API access,Login Gst portal,click profile icon,click manage API Access,after clicking window will appear here you can give 30 days valid API ACCESS then click confirm.
Step 5:Now you can check it again in the company on your BUSY Accounitng Software,download gst return again,make sure you will recieve OTP.