While filtering to add a resource to add in the rate matrix, the resource does not appear in the results when filtering to add.
The resource is not financially enabled.
Make sure the resource is fully financially enabled
If you fill in the financial properties but do not click Financially Active, you will be able to see the resource in the filter and add, but you will not be able to save the record.
Therefore, the 5 fields need to be populated.
If you then uncheck the Financially Active field, you will not be able to update the resource's record on the rate matrix.
However, you will still be able to populate a cost plan with the correct rate from the matrix