This is an expected behahiour. These columns have different data types and values in Dynamics CRM and Salesforce and cannot be mapped directly.
You need to use Lookup or Constant mapping here.
The OwnerId (Owner) is related to your Dynamics CRM organization and cannot be assigned with the values from Salesforce.
If you remove the mapping for this column in the task, the necessary OwnerId will be inserted automatically based on the used Dynamics CRM connection.
Also, you can query all possible OwnerIds via Query Tool (SELECT "OwnerId", "Name" FROM "owner") and use the necessary value in import as constant value.
If you have Owner in Salesforce with the same name as in Dynamics CRM, you can try using the Lookup mapping: https://skyvia.com/resources/docs/index.html?lookup_mapping.htm
For example, something like this:
As for Relation mapping, it is recommend to use it when you have several tasks in one package, i.e. Accounts, Contacts, etc. We recommend you to look at the predefined package "Import Salesforce data to Dynamics CRM" in the Gallery:
Also, take a look at Skyvia docs: