Recently my customers are complaining they are getting the emails with the invoices not available to view. Is there a way this can be sorted out please?

Thank you for letting us know of this issue, @sugarmoodcakes-g. I'll point you in the right direction for support.

We have an ongoing issue for QuickBooks Self-Employed (QBSE) where your customers are unable to view the invoices you send out. I suggest you contact our Technical Support for assistance. They can add you to the list of our affected users, and you'll receive a notification email as soon as an update becomes available. 

See this article for the steps: Contact QuickBooks Self-Employed Support. Please take note of their operating hours, so you know when they're available. 

A workaround I can provide is to export your invoices as a PDF file from the Invoices menu. You can then use your email to send them out to your customers.

Feel free to get back to me in the comments below if you have other questions about invoices in QBSE. I'll be around to help you out. Have a nice day!