What version of Premiere Elements are you using and on what computer operating system is it running?
You cannot open one Premiere Elements project file (project.prel) in another Premiere Elements project.
And you cannot open Premiere Elements Windows project files in Premiere Elements Mac or vice versa.
Two possibilities for Premiere Elements projects (same computer operating system)....
a. Export your TImeline content from one project to a file format consistent with the second project's project settings and import that export into the second project .
b. Windows Only. Do a Copy/Paste Insert from one project to another using ClipMate in Conjunction with Premiere Elements.
Please review and consider and supply full details of the projects and files involved if you need help with the settings for "a".
Do not hesitate to ask if you need clarification on the above.