Regarding Unpivot Table

Hi,

Pretty simple question, I have a subset of my data that looks like this:

I want to use the data as part of a Gannt chart or timeline visual, so I need all of the dates stacked into one column, as per the below:

These data are located in a sharepoint. When I select the columns I want to transform and then click "unpivot selected columns" I get the error below:

DataSource.Error: We couldn't parse OData response result.  Error: A value without a type name was found and no expected type is available. When the model is specified, each value in the payload must have a type which can be either specified in the payload, explicitly by the caller or implicitly inferred from the parent value.
Details:
    DataSourceKind=SharePoint
    DataSourcePath=https://xxx

Any ideas how to resolve this? Would it be helpful to have a transform step to copy the data I want to visualize to a new table stored within BI, and then only pivot that? How would I accomplish this?

Thank you!

Steve