Whenever you encounter 404 in Projectopia, such as 404 when trying to view an invoice or quote. This happens because WordPress have not build your permalink. You can do so by flushing your permalinks and let WordPress update them.
You will need to flush your permalinks by going to WordPress admin -> Settings -> Permalinks and click on Save Changes.
Next, proceed to clear any cache plugin or server cache’s cache content, as well as your browser history, this will make your browser load updated content.
Your content should be loading properly now.
Therefore, whenever you encounter 404, you will need to flush your permalinks.
Sorry for these inconveniences, but this is how WordPress works.