The two values on each row must be from the same subject.
Prism computes the Bland-Altman plot from raw data. Don't enter the differences and means, and then run the Bland-Altman analysis. Prism computes the differences and means.
Each column must have a value determined separately (in the same subject). If the value in one column is used as part of the determination of the other column, the Bland-Altman plot won't be helpful.