I tried creating a test material using a thermal conductivity of 0.5 W/m-K and can do that without any issue:
Generally a dot helps in specifying the decimal value unless you have different settings at your end. A comma on the other hand is not recognized and 0,5 will be read as 5. Can you please try with a dot again and do let me know if it helps. Also, please let me know what language you are using.