I know how challenging it is when you can't save an invoice with a blank SST column, azari-kamal. I'm here to help you with that.
The system automatically prompts you to enter SST on your items. I'd suggest creating a dummy one with a zero sales rate. This way, you'll be able you successfully save the invoice.
Here's how:
Go to the Taxes menu.
Click the Add tax button, then select Tax rate.
Enter a dummy Tax name.
Enter a description for your SST tax.
Set the Tax agency to SST.
On the Sales rate field, enter 0.
Add the other necessary info, then click Save.
Additionally, I'm adding these articles that'll give you more details about managing sales tax in QuickBooks Online: