I am missing the Memory category tab in Task Manager.
How can I make it re-appear? Or what can I do to add/remove a column?
The preceding answers are fine for the memory columns in the Processes tab, but not for the "missing memory category tab".
The OP seems to be looking for a "Memory" tab, alongside "Applications", "Processes", "Services", etc.
There isn't one.
What you're looking for is under the "Performance" tab. Go there, then click "Resource Monitor". There you will find your "Memory" tab.