Connecting my MacbookPro to a older iMac display.

The 2020 MacBook Pro does not support using the iMac as a display in Target Display Mode.

carefully read > Use your iMac as a display with target display mode - Apple Support

If you want a second display for the MacBook Pro, then you will need an external display.

see > Use an external display with your MacBook Pro - Apple Support

Also:

...I would like to connect to a iMac (Retina 5K, 27 tum, sent 2014)

a Retina 5K 27" has to be Late 2014, so is also too new to work with TDM.