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.
Reason for the Error
If your report does include a Lead/Contact ID field, this error message may be appearing because the Act-On user connecting to your Salesforce account needs to be set to Classic mode.
How to Resolve
Report is missing Lead ID or Contact ID
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
User is in Lightning Mode
Salesforce is currently making changes to Lightning and Classic access. Act-On is implementing a long-term solution to this issue, but for now you can resolve this error by disabling Lightning auto-switch by using a permission set. Make sure that your Act-On user that connects to Salesforce is part of this limited permission set.