In version 5.7.0.19, there is a bug that does not recognise the Constituent ID field mapping for soft credits and requires a Linked Constituent mapping. (See screenshot below.)
As a workaround, you can map the ID field to an Individual Constituent Relationship (you'll need to map to all required fields, but you can set them to not update new or existing Constituent records). You can watch a video showing how this is done here.