Unable to find object "textbox" does nto exist - desktop

What's the code? Give us as screenshot of your keyword test or a copy/paste of your script code.  Is this a mapped object?  Are the mapped properties correct?  If you step through the code, does it work?  Has the application under test changed since the code was written?

There are any NUMBER of reasons why an object will not be found by TestComplete.  But without more information, it's all just guess work.