Fedora-28 box:

Joined to ad using 'net ads join -U <>'
# net ads testjoin
Join is OK

# samba-tool domain level show
ERROR(ldb): uncaught exception - ldb_search: invalid basedn '(null)'
  File "/usr/lib64/python2.7/site-packages/samba/netcmd/__init__.py", line 176, in _run
    return self.run(*args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/samba/netcmd/domain.py", line 1055, in run
    scope=ldb.SCOPE_BASE, attrs=["msDS-Behavior-Version"])
# samba-tool domain level raise --domain-level=2012_R2 --forest-level=2012_R2
ERROR(ldb): uncaught exception - ldb_search: invalid basedn '(null)'
  File "/usr/lib64/python2.7/site-packages/samba/netcmd/__init__.py", line 176, in _run
    return self.run(*args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/samba/netcmd/domain.py", line 1055, in run
    scope=ldb.SCOPE_BASE, attrs=["msDS-Behavior-Version"])


On Window's 2008 AD:
PS C:\Users\root.ATEST> $dse = ([ADSI] "LDAP://RootDSE")
PS C:\Users\root.ATEST>
PS C:\Users\root.ATEST> # Domain Controller Functional Level
PS C:\Users\root.ATEST> $dse.domainControllerFunctionality
4
PS C:\Users\root.ATEST>
PS C:\Users\root.ATEST> # Domain Functional Level
PS C:\Users\root.ATEST> $dse.domainFunctionality
4
PS C:\Users\root.ATEST>
PS C:\Users\root.ATEST> # Forest Functional Level
PS C:\Users\root.ATEST> $dse.forestFunctionalit

Even # man samba-tool does not guides how to use 'domain level show|raise' correctly. What are prerequisites of using it.