Either you use $upper with the extension comparison in UPPERCASE
$if($eql('TTA',$upper(%_EXTENSION%)),$repl(%TITLE%,' TTA',' '),)
or you use $lower with the extension comparison in lowercase
$if($eql('tta',$lower(%_EXTENSION%)),$repl(%TITLE%,' TTA',' '),)
Do you want to replace the tta string in all variations like tta, tTa, tTA, TTa, ttA, TTA?
Or what case should be ignored?
Later added warning:
Please don't use this abbreviated syntax!
As found in this bug report, the abbrevation of $repl instead of using $replace can result in unwanted changes. Always use $replace as stated in the help.