Please add this editor object in your column and it will work for you
editor: {
type: "select",
options:['Yes','No']
}
Your updated snippet code-
{ title: "CSSP Accepted", editable:true, width: "8%", align: "center", dataIndx: "CSSPaccepted", dataType: "string",editor: { type: "select", options:['Yes','No'] }, filter: { crules: ruleR } },