Can you use datalist with HTML textarea?

No, because datalist by definition associates with an input element. Moreover, selecting an item drom a datalist means replacing the entire value of the associated input element, instead of appending to it, which normally be the idea if you used predefined alternatives for a textarea element.

There is nothing illogical with the idea of a pre-made list of alternatives to use for text area input, but there is nothing for it in HTML at present (or being planned, as far as I known). It can be coded rather simply in JavaScript, though.

You can have a list of items, for example as a ul list, or even as a select element. You would then add a little JavaScript that causes the item text to be written or appended to the textarea element e.g. when an item is clicked on or a selection is made from a select list

There is normally no reason to have a single-line text input field as a textarea. It’s possible, just not useful, except perhaps in very special cases. Note that <textarea rows=1 ...> lets the user enter any number of lines, it just makes that very inconvenient.