Client players are unable to move their characters in build but are able to in PIE

Development Programming & Scripting
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4

That means there are less than 3 players and the function ends prematurely.
Move

if (GEngine) GEngine->AddOnScreenDebugMessage(-1, 20, FColor::Cyan, FString::Printf(TEXT("%i TeamCharacters"), TeamCharacters.Num()));

Above if(TeamCharacters.Num() < 3){} to see how many TeamCharacters there are before you enter the if statement.