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Please go to Excel > File > Account > Product Information, check the update channel, is it Semi-Annual Enterprise Channel?
I do not find any announcement about the release date of LAMBDA function for Office 365 wuth version 2008.
Based on my research and test, LAMBDA function is included in Microsoft 365 apps with the Beta Channel from Version 2012 (Build 13530.20000).
Currently, this function is still not avaliable in Version 2103 (Build 13901.20462) with the Current Channel, this update channel receives new features more frequently and faster than Semi-Annual Enterprise Channel.
So I think to use this function under Semi-Annual Enterprise Channel, it needs quite some months, even longer.
If needed, you could try to join the Office Insider Program and choose the Beta Channel to get the LAMBDA function in Excel.
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