The Google Spreadsheet looks like can only select one value in the dropdown list.
Is there any way to select 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:
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