Creating Indexed View GROUP BY Epoch Date
I think there's some misunderstanding about what you're attempting to do here.

Since your current design is to return all 24 rows from the base table, presumably all the supplementary fields are returned as well (to display in a grid, or something).

In order to fully aggregate the Value column, all the supplementary columns cannot be included in the SELECT list. Alternatively, if those columns are included in the GROUP BY clause, the view would represent only a partial aggregation, as there would be one row for each unique combination of the columns in the GROUP BY column list.

The only way I see something like this being useful is if the supplementary columns aren't included in the view, and there is some other process that requires only the daily aggregated values, without the base row data. Such a view could be defined like this:

CREATE TABLE [dbo].[BaseTbl]
(
    ColRowID bigint NOT NULL,
    AggregateID int NOT NULL,
    Epoch int NOT NULL,
    CustomerID int NOT NULL,
    TypeID tinyint NOT NULL,
    ErrorID smallint NOT NULL,
    Value int NOT NULL,

    PRIMARY KEY CLUSTERED(Epoch, CustomerId)
);
GO

CREATE VIEW [dbo].[ixvw_AggTbl]
    WITH SCHEMABINDING
AS
    SELECT
        t.Epoch / 86400 AS EpochDay,
        CustomerID,
        TypeID,
        SUM(t.Value) AS TotalValue,
        COUNT_BIG(*) AS __RowCount
        FROM [dbo].[BaseTbl] t
        GROUP BY
            t.Epoch / 86400,
            CustomerID,
            TypeID;
GO

CREATE UNIQUE CLUSTERED INDEX IX_ixvw_AggTbl
    ON [dbo].[ixvw_AggTbl](EpochDay, CustomerID, TypeID);


Unfortunately, you can't go farther and convert the EpochDay column to an actual date within the indexed view because DATEADD is non-deterministic (see Aaron's comment below for why), so you'd have to convert it in the actual SELECT query against the view. But that's not too difficult.

In any event, as I said before, I'm not sure how useful this would be for your specific application.