How do I print when option is greyed out on iMac?

• What app are you attempting to print from?
• If the printer is set up correctly and the app allows printing, then you can try resetting the Print System by going to  > System Settings > Printers & Scanners, and Control Click the printer to select Reset Print System.

If Mac Jim ID's suggestion does not solve your issue then I would like to ask if you are using the Mac's Mail application, or webmail when trying to print?

If using webmail, there is one more layer of potential complication that is introduced when printing what is essentially a portion of a web page through a web browser. Because of this, most webmail interfaces provider a link or icon specifically to print just the email content instead of the entire web page (which includes sidebars, headers, etc.). If you are having issues printing from webmail, I would recommend first closing your browser, open the browser again, open the email again, and then try to print again. Browsers often need to be closed and reopened to complete required updates. When in this state, they often prevent some functions from operating normally until the pending update has been completed. Therefore, simply closing and reopening your browser could solve the issue if your Mac prints normally otherwise.