Acknowledgement is a noun that means “recognition” (e.g., “Phil’s bonus was in acknowledgement/recognition of his successful campaign”).
Acknowledgement is spelled differently in British English (acknowledgement) and American English (acknowledgment).
If you are following a style guide, then make sure you use the correct form. If you are not following a guide, then the important thing is to be consistent in your use of British or American spellings.
The QuillBot Grammar Checker can help you maintain consistent spelling in your writing.