When i want to run some RFNOC blocks in Gnuradio i faced with errors module 'ettus' has no attribute 'device3'

[sp h]

When I want to run some RFNOC blocks in Gnuradio I was faced with errors,
How can fix this problem?
My environment is Gnuradio 3.8.10
UHD:4.0.0
Ubuntu 20.04

Generating: '/home/sp/Downloads/default1.py'

Executing: /usr/bin/python3 -u /home/sp/Downloads/default1.py

Traceback (most recent call last):
  File "/home/sp/Downloads/default1.py", line 275, in <module>
    main()
  File "/home/sp/Downloads/default1.py", line 251, in main
    tb = top_block_cls()
  File "/home/sp/Downloads/default1.py", line 77, in __init__
    self.device3 = variable_ettus_device3_0 =
ettus.device3(uhd.device_addr_t( ",".join(('type=x300', '')) ))
AttributeError: module 'ettus' has no attribute 'device3'