Error Message
When importing a Salesforce report, you may see there are 0 contacts pulled in, with a warning icon indicating: 
You may be receiving this error if your Report does not include the Lead/Contact ID field which is required for a successful import into Act-On.
Solution
Report is missing Lead ID or Contact ID
If your report does include a Lead/Contact ID field, this error message may be appearing because the report includes duplicate Contact or Leads (if the report is based on Opportunities).
See our article here for instructions to add this field to your report: Adding a Lead ID or Contact ID and Owner ID in a Salesforce Report