Why won't it allow me to add my hst to my invoice

Hi @gburnett888-hotm.,

I'm glad to hear you're using the Sales Tax Centre in QuickBooks Online to accurately keep track of all sales taxes you collect and pay. Making sure you have the proper tax code and tax rate when recording transactions is important to keep your books precise. I'll be happy to assist you with this.

After you turn on sales tax in the system, QuickBooks automatically creates a list of common sales tax code for you based on the selected province. You can view the tax rate to expect for each region here: Common sales tax codes

I also recommend going to the Sales Tax Centre to ensure that your tax codes are properly set up and active. Here's how.

1. Go to the Taxes menu, then choose Sales Tax at the top. (Tip: If you use payroll in QuickBooks Online, you may need to select Sales Tax as opposed to Payroll Tax)
2. Select Manage sales tax.
3. On the Tax agencies tab, locate the tax agency associated with the sales tax rate you wish to verify.
4. You'll know the sales tax rate is active when you see Make inactive in the Actions column for that sales tax rate. 
5. If you instead see Make active, then select that for it to show up as an option when recording your transactions.
6.  

Another good troubleshooting step to resolve these kinds of situations is to navigate in a private/incognito window or use a different browser compatible with QuickBooks Online. Here's an article with the list of supported browsers: System requirements for QuickBooks Online, Accountant, and QuickBooks Self-Employed

I hope this helps! Let me know if there's anything else I can do to help.