I'm using a blog.xyz.com (Blog) and an info.xyz.com subdomain (landing pages) through Hubspot based on my www.xyz.com website. I tried to add them to google search console but was not able to do it for the info-subdomain. I used the recommended TXT file, and the alternatives with Google Analytics and HTML Tags in the header. Strange thing is that the blog-subdomain was directly verified without issues but info-subdomain won't pass. There are obviously more blog posts than landing pages but I even have internal links on my main page going to one of the landing pages.
This could be a number of issues, but from the way you asked the question, I'm wondering if the main problem is that you don't have a live page at the root of your "info" subdomain. If the exact URL http://info.xy.com/ generates a 404 response (or really anything other than a 200 OK), then it won't verify through the HTML tag or Google Analytics methods. It goes to that URL to check for those verification methods.
You could use the HTML file method by creating the html file via Design Manager in HubSpot. Just make sure you save it in the root of the domain so it lives at info.xyz.com/verification-filename.html.
I'm not sure what you mean by a TXT "file". If you're referring to the TXT record with your domain name provider, then that's not actually a file. It's a type of record you'd need to populate in your domain name registrar's management console. Your registrar should provide instructions on how to do that.
