I got the error "The role defined for the function cannot be assumed by Lambda" because i had not updated the roles "Trust Relationship" config file. I didn't encounter the timeout issues as in the linked answer in the comments.
The comments in the above answers pointed out that you need to add the following.
- Go to 'IAM > Roles > YourRoleName'
- (Note: if your role isn't listed, then you need to create it.)
- Select the 'Trust Relationships' tab
- Select 'Edit Trust Relationship'
Mine ended up like the below.
<your other rules>