Selecting Multiple Values from a Dropdown List in Google Spreadsheet

To Add to AlexG’s answer, a better and enhanced version of multi-select is found in this following link (which I tried and worked as expected):

https://gist.github.com/coinsandsteeldev/4c67dfa5411e8add913273fc5a30f5e7

For general guidance on setting up a script in Google Sheets, see this quickstart guide.

To use this script:

1. In your Google Sheet, set up data validation for a cell (or cells),
   using data from a range. In cell validation, do not select ‘Reject
   input’.
2. Go to Tools > Script editor…
3. In the script editor, go to File > New > Script file
4. Name the file multi-select.gs and paste in the contents of
   multi-select.gs. File > Save.
5. In the script editor, go to File > New > Html file Name the file
   dialog.html and paste in the contents of dialog.html. File > Save.
6. Back in your spreadsheet, you should now have a new menu called
   ‘Scripts’. Refresh the page if necessary.
7. Select the cell you want to fill with multiple items from your
   validation range.
8. Go to Scripts > Multi-select for this cell… and the sidebar should
   open, showing a checklist of valid items.
9. Tick the items you want and click the ‘Set’ button to fill your
   cell with those selected items, comma separated.

You can leave the script sidebar open. When you select any cell that has validation, click ‘Refresh validation’ in the script sidebar to bring up that cell’s checklist.

The above mentioned steps are taken from this link