The getApiUrl function is easier to read in its pre-built format here: https://github.com/pkp/ui-library/blob/master/src/mixins/global.js#L89-L113
In particular, it looks like the URL that’s being generated is causing // to appear between localhost and journal:
http://localhost//journal/api/v1/_submissions
Can you try correcting that (change localhost//journal to localhost/journal) and then try loading the URL directly in your browser to see if it works?
If it does, then we can update the getApiUrl function to account for this. If it doesn’t work, then @asmecher is probably on the right track here.