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I've just tried your exact example on Win10-64 and MMA 12 and Ta + tab in fact autocompletes to Table in my system. You may try deleting your autocompletion history.

You can find that here:

FileNameJoin[{
  $UserBaseDirectory, 
  "SystemFiles\\FrontEnd\\SystemResources\\FunctionalFrequency\\AutocompletionHistory.m"}
]