Trying to get the result with a bunch of or clauses.
Read the error message; has nothing to do with the "OR" clause.
Values are stored as "nvarchar" = string/text, you query it as it would be a numeric; of cause the could fail.
And "6,801,258,182" is much above the number range for integer = -/+ 2 G.
Query string as string =>
SELECT *
FROM SA15B
WHERE PoNo In ('6801258182');