You must identify which taxon has no common sites with others, and then remove them in order to continue your computing. Please follow this procedure to check the number of common sites between taxa: 1. Click Distances|Compute pairwise from the main menu 2. In the Analysis Preference window, select ” L: No. of Valid Common Sites” from drop down for the option “Substitutions to Include” 3. Click “compute” button to get a matrix. The number in the matrix indicates the number of common sites between sequences that are available for analysis. When this number is zero, there are no common sites. To get around this problem, you may want to eliminate some sequences. In addition, the use of the “Pairwise-Deletion” option for handling alignment gaps and missing data may also be employed.