Data Model - Relationship cannot be created

I don't have much experience with data modelling and what is possible or not.

Basically I have 2 tables. Each has different (but similar) details. They have one thing in common which is the measurement type. I don't really care for merging anything else.

I want to create a pivot or regular line chart which will show me the trend for the primary value from both tables in one chart.


However, I get the below error.
The relationship cannot be created because each column contains duplicate values ...