Date in relationship

Hi mariakn6 - Crosscheck, the if there is  any mismatched data types between the DimDate and FactTable date columns or due to ambiguity introduced by the column's formatting or data structure. 

Navigate to the Modeling tab in Power BI.
Select the column in question.
Change the data type to Date for both columns.

or you can also fix it at power query editor

Go to Home > Transform Data to open Power Query Editor.
Select the FactTable date column.
Remove or replace any non-date values.
Set the column’s data type explicitly to Date.
Apply changes and close Power Query Editor.

or 

Delete the existing relationship between DimDate and FactTable.
After ensuring both columns are of the same type (Date), recreate the relationship:
Drag the date column from DimDate to the corresponding column in FactTable in the Model view.
Set the relationship as Many-to-One (Single) and ensure the cardinality and cross-filter direction are correct.

Hope the above details helps to fix the issue.