Add Filter to Calculated Table

pbrainard had a typo, I'm writing this without testing on your data, cause I'm still not sure what is the outcome you want. I'm guessing 🙂 Try this:

CTab =
ADDCOLUMNS (
    ADDCOLUMNS (
        SUMMARIZE ( GAD_PHQ, GAD_PHQ[Client_ID], GAD_PHQ[Assess_Type] ),
        "Start Date", CALCULATE ( MIN ( GAD_PHQ[Assess_Date] ) ),
        "End Date", CALCULATE ( MAX ( GAD_PHQ[Assess_Date] ) )
    ),
    "Start Score",
        VAR _current_assess_type = GAD_PHQ[Assess_Type]
        VAR _current_start_date = [Start Date]
        RETURN
            CALCULATE (
                AVERAGE ( GAD_PHQ[Assess_Score] ),
                GAD_PHQ[Assess_Date] = _current_start_date,
                GAD_PHQ[Assess_Type] = _current_assess_type
            ),
    "End Score",
        VAR _current_assess_type = GAD_PHQ[Assess_Type]
        VAR _current_end_date = [End Date]
        RETURN
            CALCULATE (
                AVERAGE ( GAD_PHQ[Assess_Score] ),
                GAD_PHQ[Assess_Date] = _current_end_date,
                GAD_PHQ[Assess_Type] = _current_assess_type
            )
)