I have a 21,5" 2017 Imac and can not connect to MacMini to use it as target display, can you help?

Your iMac is too new. The hardware and software requirements for Target Display Model are clearly laid out in the "Related Article" you linked above your name. The LAST iMacs that could act as a monitor for another Mac were the Mid 2014 models.

Please review the sysreqs in that article for full details.

There is probably more spurious information on the internet about Target Display Mode than there is about the Loch Ness monster and a flat Earth. TDM has been incorrectly attributed as doing everything from supporting any video device ever made to parting the Red Sea.