The game finished 2-1 at Ajax Training Centre, with Rangers missing a penalty and hitting the woodwork on several occasions as they searched for an equaliser in the second-half.
Looking back on the opening match of Group A, Harkness said: "The goal just before half-time gave us a bit of belief that we could get something out of this game.
"When we went in at half-time the message was to make it more of a men's game and challenge them physically and we did that in the second-half it showed they couldn't cope with it and we started playing a lot better and creating a lot more chances.
"It just comes down to the last bit that we really need to work on, when you are playing against good teams like this then they showed that they took their two chances very well and if we slack on that then we don't get the results.
"We need to look back on it and work on it for next week when we play Napoli, which will be another challenging game similar to this. If we work on what was missing in the first-half hopefully we will be able to get a result."
Harkness feels being a part of the competition can help benefit the players’ development alongside their domestic Lowland League campaign.
He continued: "It is a different challenge from the Lowland League so it is more technical and it is a game that you want to be playing so it makes you a better player.
"It is what you need if you need to take that next step, the Lowland League to the SFPL is a jump but this narrows it a little bit and hopefully if you play well you get recognised and take the jump.
"Looking at this performance, we are looking into Napoli and thinking we have a good chance of getting a result at home. Hopefully we can turn it around and get those three points."
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