Power Platform Community Forum Thread Details

I'm working in a PowerApps dev environment. I've been trying to update a custom Dataverse file called Contracts. I have:

Downloaded an excel file which is an extract from Contracts. It includes the 3 reserved fields, with field number 1 being the GUID.
I've amended the downloaded file by filling out some of the blank fields offline.
I've tried to upload the file again using the import wizard to update the existing records based on matching GUIDS.
I've mapped the Guid field on the downloaded file to the guid field on the Dataverse file, having read from the Microsoft documentation that the matching GUIDs will be interpreted as the signal to update existing records rather than add new ones.
The target file has no alternate keys defined.
With or without duplicate detection, I keep getting errors as below:

I've tried every combination of mappings I can think of, and nothing makes it work properly. I'm obviously missing something very fundamental.

Begging the community: Please tell me what I am doing wrong! How exactly should the wizard be configured to update in place rather than default to create records?