QuickBooks Online (QBO) is GST ready. With this, your invoices should already include the 18.0% GST (18%) option in the Tax category column. When you're unable to select the said option, your browser may be currently full of frequently accessed pages. Thus, causing some unusual system responses. To isolate this, you can perform some troubleshooting steps. You can start by logging into your QuickBooks Online (QBO) account using a private browser (incognito). Here's how:
Press Ctrl + Shift + N (Google Chrome)
Ctrl + Shift + P (Firefox)
Control + Option + P (Safari)
Once signed in, go to the + New icon to create an invoice. If you're able to select the 18.0% GST (18%) option under the Tax column, return to your default browser and clear its cache and cookies. This will remove all the older data that causes viewing and performance issues. However, if the issue persists, I'd recommend using other supported browsers.
With your GSTIN added in QBO, your invoices will need to be GST-compliant. You can customise your invoices to include GST-specific information. For the step-by-step guide, you can check out this article: Customise invoice for GST.
Additionally, in case you want to know more about what should be included in a GST-ready invoice, you can refer to this article: What is a GST-ready invoice?. It contains sets of rules that an invoice by a supplier must-have.
Please let me know how it goes. You're always welcome to comment below if you have other concerns or follow-up inquiries about monitoring GST in your sales transactions in QBO. I'm just around to help. Take care always.