why cannot handle a non-unique multi-index!

the error:
INFO:nixtla.nixtla_client:Validating inputs...
INFO:nixtla.nixtla_client:Preprocessing dataframes...
error: cannot handle a non-unique multi-index!

After I checked the indexes of the two datasets, the indexes were unique, but I still got an error。

Are there any special requirements for the dataset format?

（The example I made is similar to the example of electricity tariffs for different countries provided in the article, and I did it after this example.）