Hi! Can you help me please? I don't understand why the circuit doesn't work, but it works when the output of timer 555 is also connected to the oscilloscope, although it's the same thing.

 

"Doesn't work" tells us little about the problem.
Is this strictly a simulation?
Can you be more specific about how it's not working?

 

Yes, the simulation doesn't work. I also put a resistor on the output of timer 555 and now it works, but I don't understand why.

 

You appear to be having convergence problems, which can come and go with just a slight change in the circuit.

Try the simulation with the .uic Spice command to "use initial conditions" which skips the initial DC operating point solution, and often solves the problem.

Do you know how to enter Spice commands in your simulator?

 

The circuit is OK, but not a very efficient use of components. It can only flash one color at a time, you could do that with a single pole 4 position switch.

 

Did you remember to put a logic low on the Not-G pin (Enable pin), pin 1 on the package I am looking at?

CMOS inputs left floating in real life produce unpredictable results.