UNABLE To ADD Or DROP A Constraint - SQL Complains!

Hi all, I am trying to create a CONSTRAINT but for some reason T-SQL does not allow me to do that.

When I try to DROP a CONSTRAINT it says:

Msg 3728, Level 16, State 1, Line 13'DF_TBL_SyncTable_DEVUK' is not a constraint.Msg 3727, Level 16, State 0, Line 13Could not drop constraint. See previous errors.

When I try to ADD a CONSTRAINT it says:
Msg 1781, Level 16, State 1, Line 14Column already has a DEFAULT bound to it.Msg 1750, Level 16, State 0, Line 14Could not create constraint. See previous errors.


For some reason I can't win here. Can't drop it nor can I create one. Any solution?




Code Snippet
ALTER TABLE TBL_SyncTable DROP CONSTRAINT DF_TBL_SyncTable_DEVUK
ALTER TABLE TBL_SyncTable ADD CONSTRAINT GOD_TBL_SyncTable_DEVUK DEFAULT 2 FOR DEVUK


Thanks for comments + suggestions.

*UPDATE*

I am trying to use the following code to check if the SCHEMA exists but still no luck. For some reason when I create it and wrap an IF statement around it, it doesn't detect the SCHEMA. Is something wrong with my code?






Code Snippet

IF EXISTS(

SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE CONSTRAINT_SCHEMA = 'dbo'


AND CONSTRAINT_NAME = 'DF_TBL_SyncTable_DEVUK'


AND TABLE_NAME = 'TBL_SyncTable'

)
SELECT * FROM TABLE_1
ELSE
ALTER TABLE TBL_SyncTable ADD CONSTRAINT DF_TBL_SyncTable_DEVUK DEFAULT 2 FOR DEVUK