Hi Kelsey,
The phenotype labeling file should have a .cls extension rather than .txt, so you might want to double check. That said, there are known issues when a file name or directory path contains special characters, most commonly spaces or dashes. If this is the case with your file, renaming it or any directories holding it might solve the issue.
Did GSEA give you any error message or detailed error log? You can see more detail by clicking the [+] icon in the lower left of the frame or by looking at the gsea-0.log file in the gsea_home folder in the user's home directory. You might send that along as an attachment if the above doesn't help.
Regards,
David