According to the official documentation, the following are the requirements for using relationships:
1) When relating tables, the fields that define the relationships must have the same data type.
Changing the data type in the Data Source page does not change this requirement. Tableau will still
use the data type in the underlying database for queries.
2) You can't define relationships based on geographic fields.
3) Circular relationships aren't supported in the data model.
4) You can't edit relationships in a published data source.
5) You can't define relationships between published data sources.
6) Your workbook must use an embedded data source for you to be able to edit relationships and
performance options in the Data Source page in Tableau Online or Tableau Server.
Reference: https://help.tableau.com/current/online/enus/datasource_relationships_learnmorepage.htm
