manu22 As already discussed, there are many things that can go wrong: On the software side the image was not downloaded correctly or not properly written to the SD. Or on the hardware side a damaged SD card or even a defective RPi. Starting again with a new good quality SD is a good idea.
There is one thing which was not mentioned: low voltage. Weak power supplies not only lead to temporary erratic behaviour, they can also damage the SD. How do you power your RPi? Do you know its capacity? From RPi 3B which needs 2 Ampere the consumtion has increased to 3 Ampere for RPi 4B. The signal is the red power LED. It should be rock steady.