Do you refer to a codemeter module with a "plugin"?
You can use both 19 and 21 with an updated 21 codemeter dongle.
If you have installed 19, don't uninstall it, just install 21, update the dongle, and they both work, at the same time even. I believe this the preferred method. They actually say you should not install older ArchiCAD versions on top of the newer installations, but allways vice versa.
AC25, Rhino6/7+Grasshopper, TwinMotion • Mac Pro 6,1 E5-1650v2-3,5GHz/128GB/eGPU:6800XT/11.6.5 • HP Z4/Xeon W-2195/256GB/RX6800XT/W10ProWS