Hi Masih-
For the April Public Preview, you will need to go through the process of requesting capacity for your subscription so that you specify the region(s), VM size(s), and number of cores that you need assigned to your subscription for each VM size. If you've already gone through this process for the April Public Preview and you still don't see VM sizes enabled to select for your region, then I recommend logging a support ticket to have the engineering team to investigate further to see if there is another issue causing this.
One other point I want to mention, is that the Small GPU Visualization size is currently only supported for Windows, as shown in this table: https://docs.microsoft.com/en-us/azure/lab-services/administrator-guide#vm-sizing. If you select an Linux image, you will see that the Small GPU Visualization size is disabled.
Thanks,
Nicole