check if one column value equals another column value

Your screen shots are very small. Can you copy and paste the formula as text, and also show us the relationship between the two tables?

Regarding switching from ONe-to-Many to Many-to-One:

Power BI will inspect the cardinality of the two columns when you drag and drop a join between them. Using my example above, if I drag from StateID of my State table to the StateID of the City table, Power BI will create a One-to-Many relatonship with State table shown at the top, and City table at the bottom. It mean: For every ONE State ID (in the top table), that value can show up MANY times in the City table (bottom). But if I drag the other way around (from City to State tables on State ID), my relationship will be Many-to-One (MANY State ID's in City table (top) can show up ONE time in State table (bottom) ). It's the same thing really.

It's like this: ONE Parent can have MANY children (one-to-many) or MANY children can have the same Parent (Many-to-one).