What computer operating system is your Premiere Elements 12 running in?
And what computer operating system was your Premiere Element 10 running in?
If 10 was on Mac and you have 12 on Windows or vice versa, that is a no go for opening a version 10 project in Premiere Elements 12.
Another point, you cannot import a project.prel (any version) into another project.prel (any version). So, if you have opened
a Premiere Elements 12 new project and are trying to open the Premiere Elements 10 project.prel file into the Premiere Elements 12 project.prel file, no go.
Typically it is advised to finish a project in the program in which it was created. But, if you right click your Premiere Elements 10 Windows project.prel file and selected Open With Premiere Elements 12, that would be expect to open the version 10 project.prel in Premiere Elements 12.
Please review and supply further information as necessary.