CS5 Design Premium no longer working

CS5 wasn't built to run on Sierra, High Sierra or Mojave.  Those legacy CS installers need to be run from the Terminal.

https://helpx.adobe.com/creative-cloud/kb/install-creative-suite-mac-os-sierra.html

As of this writing, CS5 and CS6 are on life support. But they can still be activated providing you have:
1. your original media (disks or downloads) you received at purchase and a way to read them,
2. a legitimate serial number for that version, language and platform (Win/Mac) and if it was an upgrade, a qualifying serial number from an older product,
3. an older operating system that can support legacy 32-bit apps.

Legacy Creative Suite is discontinued & unsupported. Use it at your own risk.

Beware of fake Adobe reps who may contact you privately offering to sell you software or take control of your computer. It's a scam, run away! Real Adobe employees will never reach out via Outlook, Yahoo, Gmail or Skype.
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3 Easy Ways to Identify Genuine Adobe Staff
https://tinyurl.com/10791730

you can always secure your data > wipe your computer > reinstall your os > reinstall your programs.

if you don't want to do all that, there are several other things you could do to uninstall, clean and reinstall but because you've explained nothing about the current problem, it's not possible to assess the chances of success, for example, with using the cleaner.