Transparency Window Missing Mask Function

[Matthew27588859y69k]

I'm trying to follow tutorials on making an opacity mask, but the tutorials all show that the Transparency window has a mask function in it, where you can link objects and their gradient masks, like this:

This doesn't exist in my version of Illustrator. My transparency window looks like this:

I'm using the latest version of Illustrator and have searched the UI and the mask function appears to be missing. Was this functionality removed? If so, how can I apply a gradient mask to an object now?

[Monika Gause]

Does a reboot solve this?

[Matthew27588859y69k]

Nope.

[Larry G. Schneider]

Does it still look the same if you select Normal instead of Screen from the dropdown menu?

[Matthew27588859y69k]

Yep. Blend mode doesn't matter.

[Matthew27588859y69k · Accepted solution]

I found the solution. The masking dialogue was revealed by clicking these miniscule arrows next to the Transparency tab title. 
Unnecessary. 

[Simona_Ilieva2513]

In my case in AI 2021:

Go to transparency mask and to the three lines and click ----> Show Thumbnails and it will appear.