Check that the Project Code does not have a space before or after it, as this affects how the system searches for it. In the Sales Transaction Detail screen, the Project field will not find the project if the code has an extra space. Select the code manually by clicking the magnifying glass and typing the Project Code.
If the Project Code has already been used, you cannot amend it directly. Instead, set the current code to Completed and create a new one without the leading or trailing space.
Alternatively, use the Project Code as currently set up.