column can only enter letters of the alphabet

You need to add a CONTRAINT on that column something like this:
ALTER TABLE dbo.YourTableName ADD CONSTRAINT AlphaOnly CHECK (PATINDEX('%[^A-Z]%', YourColumnName) > 0)

HI
create table datosPersonales (nombre varchar(100))
go
ALTER TABLE datosPersonales
      ADD CONSTRAINT nombre CHECK (PATINDEX('%[^A-Z]%', nombre) > 0)
      
insert into datosPersonales values ('ABCD')
i have this error
Msg 547, Level 16, State 0, Line 1
The INSERT statement conflicted with the CHECK constraint "nombre". The conflict occurred in database "AdventureWorks", table "dbo.datosPersonales", column 'nombre'.
The statement has been terminated.