My Office 11 is not working. it was one time purchase

I have the same issue on my late 2013 version iMac, with Office for Mac 2011 loaded. This no longer works after the latest os upgrade to Catalina.

I understand I have to now purchase a new version of office (e.g. Microsoft Home and Student 2019), but Do I have to uninstall the old version first.,before loading the new version on to my Mac?

Office 2011 does not work on MacOS 10.15 Catalina. You need to purchase Office 2019 or subscribe to Microsoft 365.