did you run the script that maya generated for you?

Quote

// One or more duplicated edges were present.
// Repair the duplicate edges by selecting the polygon
// and issuing the following command, then try again:
select .e[985] .e[843] .e[771] ;string $v[] = `polyListComponentConversion -fe -tv`;select $v;polySplitVertex;polyMergeVertex -d 0.0 -tx 1; //

this tells you to run the

select .e[985] .e[843] .e[771] ;string $v[] = `polyListComponentConversion -fe -tv`;select $v;polySplitVertex;polyMergeVertex -d 0.0 -tx 1;

part of the message in the script editor and then do the conversion to sub d again.