What datasource are you using?
I have the same issue with one site on my server but it only affects BigQuery datasources.
I have had a support call with Tableau and they cannot find any issue (analysed log files, settings etc). The only way I was able to resolve this bug was to back up the site...delete it and rebuild it from the back up
All my subscription schedules were disabled, that is why i am not seeing subscribe button. I have created a new subscription and enabled it. I am now seeing subscribe button.