I read that I can mix different series hardware. I tried, but it doesn’t work?

It's complicated.

You can use 11-speed derailleurs with 10-speed shifters and the other way around, and you can use 11-speed di2 with other 11-speed Di2, as long as you stick to a number of rules - don't mix MTB and road/gravel, for example.

Also, you may have to update your component firmware before it'll recognize the different generation / series hardware.

Have a look at the pages on how to connect your bike to a windows computer and how to update Di2 firmware for more information on this subject.

Note that you can not use 11-speed components with 12-speed Di2, except for the SW-R9150 satellite shifter and some TT/tri shifters.