Cannot enable change tracking on table ''. Change tracking requires a primary key on the table.

Before going to error, let me first give a brief idea on what change tracking is.

As per Microsoft "Change tracking is a lightweight solution that provides an efficient change tracking mechanism for applications. Typically, to enable applications to query for changes to data in a database and access information that is related to the changes, application developers had to implement custom change tracking mechanisms."

More information on Change Tracking can be found on MSDN Here 

Now About this error, As the error message is self explanatory, you cannot enable or track a table using change tracking without a primary key on. This is because the values of the primary key column is only information from the tracked table that is recorded with the change information. These values identify the rows that have been changed. To obtain the latest data for those rows, an application can use the primary key column values to join the source table with the tracked table.

To Fix this solution, Just add a primary key column to table if not existing or if any of the existing column can be made primary key.