how to avoid login after system asleep?

Sleep is unnecessary and should be disabled. Shut for long absences/idle overnights (i.e. do not manual-sleep), and for regular operation, use the settings exactly as shown below (reboot after changes) should fix.

Additionally, avoid manual sleeping your Mac and use the Lock Screen settings to switch the display off on timer:

This is done in System Settings. Click the Login Password link on the left side. The window that opens will show a button that allows you to Automatically log in after a restart. Make sure its is on.

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If it isn't what hcsitas points out, is Automac login as... setup?