Macbook Air doesn't recognize Imac as external display and vice versa

Hello All I have a 2013 Macbook Air with an Intel HD Graphics 5000 1536 MB graphics card. I recently bought a 2017 imac intending to use it partly as an external display. Radeon Pro 570 4 GB graphics card. I bought a monoprix mini display port (Macbook Air) to USB-C (iMac) adapter. Neither computer recognizes external monitor. Tried everything I could find. Both running Catalina. Ideas?