Go back to Pro capacity after using Fabric capacity

[ThoStart]

Last week i used the fabric trial. 

In workspace settings -> premium, i set up the fabric capacity (instead of pro)

I had to make an azure trial account and made fabric capacity server there.

Now my the trial of azure is finished ( I can not open any report) and i want to switch back to pro capacity in the workspace. 

The button for 'apply' is switched off. as shown in the picture below..

How can i fix this?