Are the data paired?

The two values on each row must be from the same subject.

Are the values entered into the two columns actual results of lab results?

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.

Are the two values determined independently?

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.