Create an in-cell dropdown or numeric/text data-validation rule that restricts what values a range of Google Sheets cells can hold.

Documented steps

1. POST https://sheets.googleapis.com/v4/spreadsheets/{spreadsheetId}:batchUpdate using OAuth scope https://www.googleapis.com/auth/spreadsheets.
2. Add requests[0].setDataValidation.range = GridRange for the target cells (zero-based, half-open indexes).
3. For a fixed dropdown list, set rule.condition = {type:"ONE_OF_LIST", values:[{userEnteredValue:"A"}, {userEnteredValue:"B"}, ...]} — the cell's value must be in the list of condition values.
4. For a dropdown sourced from another range, set rule.condition = {type:"ONE_OF_RANGE"} with a values[] entry whose userEnteredValue references the source range (e.g. "=Sheet1!B2:B10") — the cell's value must be listed in that range.
5. Set rule.strict (true rejects invalid input) and rule.showCustomUi (if true, List conditions show a dropdown) and optionally rule.inputMessage (helper text shown while editing).
6. Send setDataValidation with only a range and no rule to clear existing validation from that range.
7. Docs: https://developers.google.com/workspace/sheets/api/reference/rest/v4/spreadsheets/request | https://developers.google.com/workspace/sheets/api/reference/rest/v4/spreadsheets/cells | https://developers.google.com/workspace/sheets/api/samples/data