Why do I need a ground when simulating a circuit? I thought voltage was relative between two nodes!
You are absolutely correct: Voltage is only defined between two nodes.
In many electronic circuits there is a constant-voltage* power supply that connects to many parts of the circuit. By convention, the more positive terminal of the power supply is labelled "V+" or "Vcc" or...
By convention, the more negative terminal of the power supply is called "ground."
By convention, we often do not draw either the V+ net or the ground net in circuit diagrams. Instead we connect things to a V+ symbol or, to a ground symbol.
And finally, by convention, whenever we talk about the voltage at any point in the circuit, we are implicitly talking about the voltage between that point and the ground net.
Your simulation tool is simply honoring that last convention. Therefore it requires a reference net called ground.
* or, some approximation thereof
the specifics of the question is:
As far a I understand I should get a curent of (I = V/R) 1 Amper. But the stimulation does not give a solution and sais I should have ground.
Why should I have ground if I have a voltage source that gives potential differences from its two sides?
It comes down to how simulators work. Simulators require a reference point and this reference point is designated by the GND symbol. internally, the engine will determine the system equation and responses against this reference.
This limitation doesn't exist in the real world because of physics.
Normally it is 'assumed' that the minus (-) side of the power supplier is 0V, so if you connect the ground to the minus side, that will be 0 V. and the plus (+) side will be 1V (GND + difference = 0 + 1 = 1)V.
If you would put the ground at the plus side, the minus side would be -1V.