I have never noticed this button either.
I have been getting this message for over a month and a half.
I even called ProSeries Support and they stated that the IRS was still not accepting 2018 returns in the 2020 filing season and I just needed to wait. That I would be notified via my software in an Alert when they began accepting returns again.
I kept trying over and over and searching for anything official saying that returns for 2018 were not being accepting and could not locate anything.
Finally, today, I ran across this post and low and behold, once I "refreshed installed products" I updated for about 5 minutes, which is an incredible amount of time, as you all probably know.
After the updates installed, I attempted to eFile my client again... viola - SUCCESS! I probably could have filed him WEEKS AGO!
Thank you to the Support Group!!!!