The request failed. Error code: 'GraphLookUpRequestFailedError'. Error Message: 'User lookup request to Graph failed lookup for user identifier:

[resolver101757]

When i drop a file in the respective document library, i get look up request to graph failed lookup for my user?  Can anyone shine any light on it.  I've included a screen shot of the flow too. Many thanks 

The request failed. Error code: 'GraphLookUpRequestFailedError'. Error Message: 'User lookup request to Graph failed lookup for user identifier: '{"@odata.type":"#Microsoft.Azure.Connectors.SharePoint.SPListExpandedUser","Claims":"i:0#.f|membership|alex*********","DisplayName":"Alex","Email":"alex**************************'.'.

the flow : 

[annajhaveri]

@resolver101757  try using "Modified By Email" from dynamic content in the assigned to parameter of approval action.

[GraemeNZ]

@resolver101757 ; @annajhaveri 
For any other users who have this same error and check this post (like I did)... I was having a dickens of a job trying to find an answer for my flow, and the forum has several different posts which are all unresolved about this issue. I finally got an answer from my friendly IT professional.

The template 'Start and wait for an approval' has a mandatory field that doesn't show as mandatory, and is hidden in the advanced options. This field is Requestor. My SP library has a column for Procedure Owner, and when the flow is triggered it sends an approval request to Procedure Owner. However it also must have Requestor specified. This could be Modified by email, but in my case we have used User email.

I hope this helps everyone else struggling with the error.

Cheers, GraemeNZ