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I use a RPI3 with the official 7" Touchscreen.
the "touch" works, but if i want to make a input, no Softkeyboard pop-up.
I install several keyboard with no luck.
Android doesn't show the virtual keyboard if it thinks that a physical one is connected. Please check your connected USB devices - maybe one of them is recognized as a keyboard.
In any case, you can enable the setting to always show the virtual keyboard. There are many tutorials on the Internet how to do that. Here is one example: https://stackoverflow.com/questions/10561760/how-to-enable-both-hardware-and-virtual-keyboards-on-android-ice-cream-sandwich/10670357