A .prel file is a Premiere Elements project file -- not a media file.
You don't open a .prel by using Add Media. You open it by going to the program's File menu and selecting Open.
What version of Premiere Elements are you using and what computer operating system is involved?
How are you trying to open an existing project....from the Welcome Screen, from within an open project with File Menu/Open Project, or something else?
Please give details step by step. As you have already been advised, you cannot import one project file (.prel) into another. But, there are other possibilities which might be inferred by the wording of your question and not enough detail.
Does this problem extend to all existing projects or just this one? Can you create a new project?
The answers are in the details. Please consider and supply more details and then we can decide what next based on the details in your reply. At that time, I can give you a customized reply to help you resolve your issue.
Any questions or need clarification, please do not hesitate to ask.
Purchased a new computer..Windows 10. successfully re installed Premier Elements and Photoshop elements, both version 12. Used product keys and all went well. Transferred over all the still photos but have not been able to transfer the completed videos that are PREL file.
Used this sequence File>open project (the PREL files show up) select one and it shows up in File name box with file of type box showing Adobe Premier Elements Program>click open and it goes to main screen but project does not load.
New box opens with heading across the top Where is the file....(super long number ending in .MTS and all the files show up again.
I click on the PREL file I want to open and it shows up in the file name box. File type box reads All supported files. ( I checked the drop down box and PREL files are NOT listed
>select and the message shows up "this type of file is not supported or the required codec is not installed.
As a note I imported an mpeg file and it loaded without any problem.