"Legacy image should be converted" - how to convert?

Hello. I am trying to install an app but I'm getting the "Legacy image should be ocnverted" message. I looked this up and all the answers said to open disk utility and to either click and drag the .dmg into disk utility - or - to open disk utility and click the 'convert' tab on the disk utility panel.. Problem is there is not 'convert' tab and I can't click and drag it into disk utility either. I really need some help to figure out how to convert this .dmg!

My OS version is 10.12.5 Sierra. ANY help is really appreciated. Thank you 🙂