As per the documentation

accountIds  string[]    CfnConfigurationAggregator.AccountAggregationSourceProperty.AccountIds.

In other words you need to pass in an array of strings like so:

const globalAggregator = new config.CfnConfigurationAggregator(this, 'globalAggregator', {
    configurationAggregatorName: 'globalAggregator',
    accountAggregationSources: [{ 
        accountIds: [awsAccountID],   
    }]
});