Voiding transactions is essential for maintaining accurate financial records, Jennut.

I replicated your concern and was able to void the sales receipt. For visual reference, I've added a screenshot below.

To resolve the issue, I recommend logging out and logging back into your QuickBooks Online account. Doing so refreshes the data and for the updates to take effect. 

If the Void button is still missing, I suggest accessing your account using a private window. This way, we can determine whether this is a browser cache-related issue. The cache helps the web pages to load quickly. However, excessive amounts can lead to problems that impact the appearance and functionality of the program.

Here are the keyboard shortcuts: 

• Safari: Command + Shift + N 
• Google Chrome: CTRL + Shift + N
• Mozilla Firefox or Microsoft Edge: CTRL + Shift + N

Once done, clear the browser's cache. Doing so deletes all the data from your browser and optimizes its performance. Using other supported browsers also helps us verify if this is a browser-specific issue. 

Additionally, I've added an article handle when invoices you send in QuickBooks become uncollectible. This way, your accounts receivable and net income stay up-to-date: Write off Bad Debt in QuickBooks Online.

I'm here to assist you with any future concerns you may have regarding managing your sales transactions in QuickBooks. I'm always here to help you out.