@Mourad Chahri You can go to the ResourceManager UI. From there you should see a nodes link on the left side of the screen. If you click on that, you should see all of your NodeManagers and the reason for it being listed as unhealthy may be shown here. It is most likely due to yarn local dirs or log dirs. You may be hitting the disk threshold for this. There are a couple of parameters you can check for this.
Finally, if that does not reveal the issue, you should look in /var/log/hadoop-yarn/yarn. Your previous comment shows you were looking in /var/log/hadoop/yarn which is not where the NodeManager log is located.
That is not a good solution as you are saying yarn does not need any healthy disks to function. You essentially disable the health check with this. There is an underlying problem with the disks and that is the reason for it being marked unhealthy.
I have a similar issue, all 3 node managers in my cluster are marked as bad with local dirs bad alerts and I can assure I have plenty of space on the YARN disk. (just 35% of the disk is used). Does anyone know of this alert other than YARN reaching its disk threshold value??