Hi @ Andy-MyCXUk
Could you please share a bit more about the error message within your PowerApps?
I did a test on my side and it worked fine.
I create two Excel table and set the data column to date format (then update two tables to One_Drive):

Then, create two data tables in powerapps:


Filter and sort data tables:
Workstack table:
Datasourcetask table:

Here are some check points you can try:
1: check the formula whether the symbol is missing or the column name is wrong;
2: determine column “completiondata” and “startdate” data type is date, otherwise the sort function won’t work correctly;
The introduction of “SortByColumns” :
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/functions/function-sort
I don’t find the way to fill Data Table row to red if the completion date is less than or equal to today () but you can do that in a gallery using the following formula:
Fill = If(ThisItem.completetionDate<=Today(),RGBA(255, 0, 0, 1),RGBA(0,0,0,0))
Hope this helps.
Best regards,
Zhen