Deleted Foreign Key Constraint causing error - sql-server

Question

I'm trying to create a FK constraint by using this auto created script.
USE [PRM.Mobile.SaaS]
GO
ALTER TABLE [dbo].[Activity] WITH CHECK
ADD CONSTRAINT [FK_Activity_Activity]
FOREIGN KEY([ParentActivityID]) REFERENCES [dbo].[Activity] ([ActivityID])
GO
ALTER TABLE [dbo].[Activity] CHECK CONSTRAINT [FK_Activity_Activity]
GO
But I get these errors:
Msg 547, Level 16, State 0, Line 4
The ALTER TABLE statement conflicted with the FOREIGN KEY SAME TABLE constraint "FK_Activity_Activity". The conflict occurred in database "PRM.Mobile.SaaS", table "dbo.Activity", column 'ActivityID'.
Msg 4917, Level 16, State 0, Line 8
Constraint 'FK_Activity_Activity' does not exist.
Msg 4916, Level 16, State 0, Line 8
Could not enable or disable the constraint. See previous errors.
Looks pretty straightforward but I don't have any FK with a name of FK_Activity_Activity anymore. Also first message being a FK constraint preventing the action and second message being that constraint is not existing is a bit confusing.
I double checked the INFORMATION.SCHEMA and sys.objects and nothing with a name of FK_Activity_Activity shows up.
These are the queries I used for checking
SELECT *
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
WHERE CONSTRAINT_NAME LIKE '%Activity_Activity%'
and
SELECT
OBJECT_NAME(object_id) AS ConstraintName,
SCHEMA_NAME(schema_id) AS SchemaName,
type_desc AS ConstraintType
FROM
sys.objects
WHERE
type_desc LIKE '%CONSTRAINT'
AND OBJECT_NAME(parent_object_id) = 'Activity'
What am I missing here?

Answer 1

The add constraint failed with the first error: the FK is not created.
The second error is from the second statement because the FK does not exist.
Everything else follows from there.
R TABLE [dbo].[Activity] WITH CHECK
ADD CONSTRAINT [FK_Activity_Activity]
FOREIGN KEY([ParentActivityID]) REFERENCES [dbo].[Activity] ([ActivityID])
To fix things so you can create the index you need to correct your data so it is consistent with the new FK before you create the index. Whether than is modifying the data in ParentActivityID or ActivityID only you can say.
But note any recursive structure needs to allow for ParentActivityID being null (unless the graph is specifically cyclic). Usually this is by making it null.