I've received a report that being unable to input Goods and Service Tax ID is under investigation. And, our Product Team is currently validating the cause of the issue.
I highly suggest contacting our QuickBooks Online Support Team so they can pull up your account and add it to the investigation (INV-42570). This way, you'll receive updates regarding the issue via email.
Here's how to get in touch with them:
Sign in to your QuickBooks Online company.
Select Help (?) at the top right.
Click Contact Us to connect with a live support agent.
Once fixed, you can then complete these steps below on how to enter a GST registration number.
Sign in to your QuickBooks Online account.
Click the Settings ⚙ icon at the right top, then select Account and Settings.
In the Company name section, click the Edit ✎ icon.
Enter your GST Registration Number in the Goods and Services Tax ID field.