The Google Sheets iOS app supports selection of items from a list and data validation on entry, but does not include the ability to add new data validations to a cell. Impressively, you also may ...

Google Sheets ... one by going to sheets.new. 2. Select the cell, or cells, where you want to add a drop-down list. 3. Right-click into one of these cells. 4. Select "Data Validation." ...