When I request a runs test as part of linear or nonlinear regression, Prism did not provide the P value.

The problem may be that your X values are not sorted. Prism is not able to compute the runs test from linear regression (and from nonlinear regression with older versions of Prism) unless the X values are in order. From the data table, click "Edit" then "Sort by X Value". Now direct Prism to recalculate the results--switch to the results sheet, choose "Change.. Analysis Parameters...", and click "OK".

When there are very few runs and lots of data points, the P value will be tiny. In some cases, Prism reported 'nan' (which means not a number) instead of a tiny P value. In other cases, the P value was left blank. This is fixed in Prism 5.03 and 5.0c.