Altium - design rule check does not match with signal integrity for impedance
The issue you are having has two reasons


Altium requires a power/ground plane defined as a reference. Unfortunately Altium is not capable of calculating the impedance on a 2 layer board with a copper pour as a reference. See this whitepaper
The stackup and track width you have used will not give you 50 Ohms unless you use track widths of 1.5mm.


Solution: make it a 4 layer board, or use a thinner board thickness