Postgres blocking UPDATE on new column?
It's a case-sensitivity issue.

"dateType" including the quotes is a quoted identifier, which implies that it retains its case (as opposed to an unquoted identifier which is implicitly converted to lower case). Quoted identifiers allows other columns of the same table named for instance "DateType" or "datetype" or other variants that differ only by case (not that it would be a good idea).

The drawback is that every subsequent SQL instruction refering to that column must use "dateType" too, with the double quotes and the exact same case. In your UPDATE, the double quotes are lacking, which is why it errors out saying that "datetype" does not exist (note how it differs from "dateType").

In practice, an identifier should be either always double-quoted, or never. A mix and match is not allowed. When an identifier is not quoted, it can be written in camel case if you fancy that style: the SQL parser will ignore the case.

See Identifiers and keywords in PostgreSQL documentation for more.