Creating new table: Too many rows error

cathoms you are using DQ and the intermediate result exceeding the 1M rows.
I guess patientdim has a lot of rows.
Maybe try to change the code to:

LegendCategories =
UNION (
    SELECTCOLUMNS (
        VALUES ( PatientDim[RaceCategory1] ),
        "Category", PatientDim[RaceCategory1],
        "Field", "Race Category"
    ),
    SELECTCOLUMNS (
        VALUES ( PatientDim[RaceCategory2] ),
        "Category", PatientDim[RaceCategory2],
        "Field", "Race"
    ),
    SELECTCOLUMNS (
        VALUES ( PatientDim[Sex] ),
        "Category", PatientDim[Sex],
        "Field", "Sex"
    ),
    SELECTCOLUMNS (
        VALUES ( CovidVaccinationStatusFact[AgeGroup 1] ),
        "Category", CovidVaccinationStatusFact[AgeGroup 1],
        "Field", "Age Group"
    ),
    SELECTCOLUMNS (
        VALUES ( CovidVaccinationStatusFact[CovidVaccinated] ),
        "Category", CovidVaccinationStatusFact[CovidVaccinated],
        "Field", "Vaccination Status"
    ),
    SELECTCOLUMNS (
        VALUES ( CovidVaccinationStatusFact[Booster] ),
        "Category", CovidVaccinationStatusFact[Booster],
        "Field", "Booster Status"
    )
)

      

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