In order to activate the Complete button in HSTE, all segments must be marked as 'saved' with a green tick and there must be no segment validation errors (red ticks).
To find any validation errors, you can open up the first (top) segment and press and hold down the TAB key on your keyboard. This will cycle through all of the segments and will stop on any that contain an error.
You will need to fix the error(s) (usually missing/incorrect tags) and then mark the segment as saved. Do this for each segment that contains an error and the Complete button should be active.
To fix tag errors, it is easiest to use the Copy From Source button to copy the source tags into the target. Before doing this, copy your translation out of the target segment and then you can paste it back in, after moving the tags.
Note: Orange ticks are irrelevant with regards to completion and only serve as warnings.