First, we need to make sure that this tax rate is active. You can follow these steps to check it:
Go to the Taxes menu.
From the Edit GST drop-down, select Edit rates.
Locate the GST @ 18.0%. Under the Action column, make sure it is On. Toggle it if needed.
If you can't find it, click the small Gear icon on top of the Action column and select Include inactive. You can also add a new tax rate by clicking the Add tax rate button.
If the same thing happens, let's perform some troubleshooting steps to see if this is an issue with the browser. Start by using an incognito window and enter an invoice from there.
Google Chrome: Ctrl + Shift + N
Safari 11 or newer: ⌘ + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Microsoft Edge: Ctrl + Shift + P
It's best to clear the cache to fix browser-related errors. Lastly, make sure that the browser is up-to-date.