Apple decided 18 months ago to only support 64-bit apps with the release of macOS Catalina.
As Adobe Photoshop Elements 2018 was released 3 years ago and contains some 32-bit code, unfortunately that version is no longer compatible with the latest macOS. You will need to upgrade to the latest Adobe Photoshop Elements 2021 release. The standard price for the upgrade is around $80.
Alternatively, you could postpone the upgrade for now and continue to work with Photoshop Elements 2018 on your old iMac instead.