I fixed the problem. the solution is here :
at the end of the page :
Hi,
is your default python version 3.x?
If yes, change in the file /usr/sbin/l4t_payload_updater_t210, line 1:
#!/usr/bin/python
to
#!/usr/bin/python2
Milan
this is another little bug of nvidia. im realizing that the software of this board is full of little unresolved bugs. they are not criticals,but they are very annoying. but the worst thing that I hate is that nvidia does not care so much about the upgrading. the nano still runs with 18.04,but for this version of ubuntu is even ended the life cycle.