[thih9]

> If you find software that doesn’t have a license, that generally means you have no permission from the creators of the software to use, modify, or share the software. Although a code host such as GitHub may allow you to view and fork the code, this does not imply that you are permitted to use, modify, or share the software for any purpose.

Source: https://choosealicense.com/no-permission/

[cube2222]

Additionally, if you accept a contribution to your repo that doesn't have a license, you as the owner of the repo actually have no right to the contributed code.

This also means that technically speaking, you can't add the license after the fact without contacting (and getting the agreement of) all contributors that have ever contributed to your repo.