I think I might have an old version of the Paclet,

Your version seems fine, I have the same:

"Version" /. PacletInformation["SemanticImport"]

(* ==> "0.0.458" *)

Try removing "SemanticImport" from your machine and the re-run SemanticImport["ExampleData/50states.txt"]. If you are on Windows, it should be here: C:\Users\*username*\AppData\Roaming\Mathematica\Paclets\Temporary. Beware, AppData is a hidden folder.

Even after getting Mathematica 11.3, SemanticImport command won't work. Even if I copy straight from the Wolfram page, I keep getting the same error.

In[3]:= data2 = SemanticImport["ExampleData/50states.txt"];
First[data2]

During evaluation of In[3]:= SemanticImport::unexpinvaliderr: Unexpected invalid input was detected.  Processing will not continue

During evaluation of In[3]:= First::normal: Nonatomic expression expected at position 1 in First[$Failed].

Out[4]= First[$Failed]

Hi,

Please download the latest (11.3) version of Mathematica.

$VersionNumber

(* ==> 11.3` *)

data2 = SemanticImport["ExampleData/50states.txt"]; 
First[%]

(* ==> "Alabama, United States"Entity["AdministrativeDivision", {"Alabama", "UnitedStates"}]"Entity[\"AdministrativeDivision\", {\"Alabama\", \"UnitedStates\"}]""administrative division" *)

Best, Mikayel