My customers are receiving the email with invoice but are Not able to open / view it why is that?

Thanks for reaching out to the Community, @coreygibson17-gm.

We have received several reports regarding this, and our Product Engineers are now working to fix it as quickly as possible. While they do, I recommend reaching out to our Customer Care Team, so they'll add you to the list of affected users (INV-57100). 

To reach them, click the Assistant button at the top-right corner. For more details, take a look at this article: Contact QuickBooks Self-Employed Support.

In the meantime, you can export the invoice as a PDF and send it manually to your customers. 

To export, here's how:

1. In the left menu, click Invoices.
2. Locate the invoice.
3. Click the Action drop-down and select Export as PDF.
4. Open the file and download them on your computer.
5. Send it manually to your customer.

Thank you for your patience while we work for a fix. If you have additional assistance with QuickBooks, let me know. I'll be around to provide further assistance. Have a nice day.