Cannot switch to desktop by pressing alt+tab in ubuntu18.04

Press alt+tab there is no desktop option, only the current application, ubuntu16.04 can be switched directly to the desktop.
I hope God will tell me how to set it up.

Mar.09,2021

use windows + D key

MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e4c903-4464d.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e4c903-4464d.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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