Hi Tom,
Thanks for your feedback, I did the following steps after your suggestion:
1. Opened the All purchase orders grid view. Default view is set to "Standard view".
2. Applied filter on "Purchase pool" field.
3. Now I can see the * (asterisk) sign just after the standard view label, but there is no option to "Save" instead there is "Save as" option.

4. I saved the view and named it "PO with Pool 10" and now the "Add to workspace" button is enabled.

Appreciate the help extended.
Cheers!
Riel