It seems to me that your strategy opens in read only mode. This is the reason you can not edit it. This happens when you open the strategy directly from the list of rules. You need to open the strategy from the change request.
In the exercise steps this is what we advised to do:
2. In the decision strategy, add three propositions form the Tablets group.
a. Open the newly created change request.
b. Open the Next Best Offer strategy. This may take a short period of time.
Can you please ensure you follow these exact steps? Than you will get the strategy automatically checked out, and able to edit.