Is anyone having trouble not being able to set up a project on an existing customer?

I Wendy-Longly, 

Thank you for reaching out on this and welcome to our community!

I have tried and created a project on an existing customer in my test file and I was able to do it without any issues. So we can better assist you with this, do you mind tell us a bit more about the experience you are having when trying to complete said task in QuickBooks Online, please?

Thank you and please let us know if you require any additional assistance.