When adding a new custom field, you can select the field type "Select" and it will be used as a "dropdown" field.
( example screenshot: hidden link )
If you've already added this field, but it is not showing as a dropdown, you're welcome to share temporary admin login details, along with the link to the page where this field can be seen.
I'll be in a better position to guide you with the next steps, accordingly.
Note: Your next reply will be private and it is recommended to make a complete backup copy, before sharing the access details.