“Premiere Pro Project Manager issue

After I finish my work in Adobe Premiere Pro, I save the project. Then, using File → Project Manager, I collect all my files into a single folder. However, when I try to open the project from the newly created folder, I get the error:

“The project could not be loaded, it may be damaged or contain outdated elements.”

What is the reason for this? I couldn’t find any information about it on the internet.