Create calculation in a table based on column from another tbale

Hi, 

I have two tables Table A and Table B which are joined by ID and have a many to one realtionship  between A and B. 

I need to create a calculated column in Table A based on some columns from Table B. I tried using the RELATED function which is giving me the below error, even though there is a relationship:

The column 'Table B[X]' either doesn't exist or doesn't have a relationship to any table available in the current context.

Any assistance appreciated.