Using older versions of Outlook for Mac in business environments
Outlook for Mac is only free for personal email accounts. To use to access a work or school account you need a paid version or 365 subscription. The message means your work account does not include a paid subscription (verify with your wrk admin). As suggested you can use the web version of outlook (office.com).
To get desktop access you can buy a personal office 365 subscription to enable the outlook app to access your work email, or use the builtin Mac mail and calendar apps.
Note: Mac mail and calendar app also work with personal outlook email accounts