Savings Plan Recommendations are only available for the accounts that have usage.
The EC2 Instance Hours report will show if the account has any EC2 usage, in this example the "payer account is selected and does not have usage:
By default the Billing Account that get's populated in the Saving Pans Recommendations is the "payer" account. Since this payer account doesn't have usage, there will be no recommendations:
If we check the EC2 Instance Hours report for another example account, we can see it does have usage:
Subsequently, in the Savings Plan Recommendations if we change the Billing Account to the account with usage, we can get the recommendations: