I don't understand clearly why the ideal sources that the value doesn't match can't be connected
Ask yourself:

For the left circuit, what will be the voltage across V1 ?


10 V ? No because V2 puts 5 V directly across V1.
5 V ? No because V1 puts 10 V directly across V1.


Do you spot the contradiction?

For the right circuit, what will be the current flowing ?


5 A ? No because I2 forces 2 A to flow.
2 A ? No because I1 forces 5 A to flow.


Do you spot the contradiction?

In these circuits there is no solution as the sources are ideal so their voltage or current must be obeyed (for example: a 5 V voltage source must have 5 V across its terminals).

For the left circuit the KVL states that V1 = V2 as the sources are in parallel, fill that in and you get: 10 = 5

We all know that 10 is not equal to 5.

For the right circuit KCL states that I1 = I2 as the sources are in series, fill that in and you get: 5 = 2

We all know that 5 is not equal to 2.