You need to include every trip you have made.
However, it shouldn’t be an issue. I would just make a note in your cover letter about the error and include a separate sheet with your printed application, listing every trip you have made.
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The application changed the travel history requirements recently as well and it no longer requires you to enter all travel history for the UK and the EEA. The only constant is change, right?!

What it requires now are the following:
1. UK travel: 3 most recent trips
2. Countries including EEA nations, Australia, Canada, New Zealand, and USA: 2 most recent trips
3. All other countries excluding the above: application allows at least 30 entries