Is it possible to uncheck the Allow NULL check box when editing the attribute table in ArcGIS Pro?

Answer

No, it is not possible to uncheck the Allow NULL check box for a specific field when there are entries in the attribute table in ArcGIS Pro. Disabling nulls in the attribute table can only be done at the time of field creation. To uncheck the Allow NULL check box when there are attribute values, recreate the attribute table in a new feature class. Follow the workaround below to recreate the attribute table and disable the Allow NULL check box in ArcGIS Pro.

Note: 
The ObjectID and GlobalID changes when the attribute table is recreated.

1. Open the ArcGIS Pro project containing the feature layer.
2. In the Catalog pane, create a new feature class. Refer to ArcGIS Pro: Create a stand-alone feature class for further instructions. The field definitions must be imported from the original feature class. Click Import and browse to its location, select the feature class, and click OK.

3. In the Contents pane, right-click the new feature layer and click Attribute Table.
4. On the ribbon, on the Table tab, click Fields in the Field group.
5. Uncheck the Allow NULL check box for the desired fields. Click Save in the Changes group.

6. Copy the features of the original feature class and paste them to the new feature class. Refer to ArcGIS Pro: Paste to a specified layer for more information. Check the Keep source attribute values check box.

Note: 
Relationship classes are not retained when feature classes are copied and pasted to another feature class.

7. Delete the original feature class.
   1. On the View tab, in the Windows group, click Catalog Pane.
   2. Expand Databases and expand the file geodatabase.
   3. Right-click the feature class and click Delete. When prompted, click Yes.