Thanks for replying.

!pip freeze | grep transformers
transformers==3.4.0

And, I still get the same error

import tensorflow as tf
from transformers import LongformerTokenizer, TFLongformerForMaskedLM


ImportError                               Traceback (most recent call last)
 in 
1 import tensorflow as tf
----> 2 from transformers import LongformerTokenizer, TFLongformerForMaskedLM
ImportError: cannot import name ‘TFLongformerForMaskedLM’