Both acknowledgment and acknowledgement appear in the English-speaking world, but acknowledgment, without the middle e, is preferred in the United States of America and Canada. Hence, Acknowledgement is preferred outside North America. These preferences extend to the plural forms, acknowledgements and acknowledgments.
In the USA and Canada, acknowledgement appears about once for every two instances of acknowledgment. The ratio is the other way around in British and Australian publications, at least the ones that make their content searchable online. So no metter where you are writing, both forms are considered acceptable, but it may be safer to stick with the one that your readers are more likely to consider correct.