I see, well just to be sure did you clone this repo: https://github.com/leekunhee/Mask_RCNN ?

And can you update Tensorflow to 2.3.

It's recommended to use the latest version of TF if you want to convert to TFLite. Actually, there is a lot of enhancements and new supported features that's why ;)

Try with the latest version of TF and keep me updated ;)