Bulk assign paths to nodes
1) Make sure you have set up a working alias for your content type at /admin/config/search/path/patterns 

2) Go to /admin/config/search/path/update_bulk

3) check "Content"

4) Submit

I have just tested this on my local dev environment by going in the db and deleting the alias to one node:

delete from url_alias where source = '/node/2940'


and running the bulk operation generated an alias for all unaliased nodes (which just happened to be the only one I deleted).

So if the bulk operation isn't working for you, it may be that you don't have a pattern set up for your content type (node bundle).

Check at /admin/config/search/path/patterns and make sure you have a working pattern set up for your content type.