You have a tblProjectHoursInvoiceList where you appear to be saving a Total. Aggregate date is normally not saved, it is calculated from transaction records when needed. Same goes for InvoiceTotal field in tblInvoice.And same for ProjectActual field in tblProject.Not sure what InvoiceDetails field is for.
Might want to revisit the tblInvoiceDetailTable Steve showed in his example.
Quantity field in tblProjectProductsInvoiceList needs to be number type. Only save product Price here if there is a chance it could be changed in tblProduct in the future.
I don't think tblProject needs to link directly to tblInvoice in Relationships builder nor the Project_ID field in tblInvoice.
Advise not to build lookups in table. Build comboboxes and listboxes on forms.
See one way to build invoice in attached db.
WELLCDB003.zip