Dynamic Column add to table from other table columns and getting error what wrong i'm doing here

alter PROCEDURE [dbo].[INSERT_CERTIFICATE_FIELD]

AS

declare @ColumnName varchar(100)

declare @field_List varchar(100)

declare @getColumnName cursor

declare @SQLFieldUpdate varchar(4000)

declare @tableName varchar(100)

set @tableName='CERTIFICATE_DATA'

---------------------------------------------------------------------------------

--get all CERTIFICATE_FIELD_NAME list from CERTIFICATE_FIELD add check all are created as field name to CERTIFICATE_DATA table

-- TO CHECK ALL FIELD CREATED HERE EXIST IN CERTIFICATE_DATA AS COLUMN

       set @getColumnName= cursor for

       select FIELD_NAME from  CERTIFICATE_FIELD

       open @GetColumnName

       fetch next from  @getcolumnName into @columnName

              while @@FETCH_STATUS=0

                     Begin

                           SET @SQLFieldUpdate='IF COL_LENGTH('+@tableName+', '+@columnName+') IS  NULL

                                                       BEGIN

                                                                      alter table '+@tableName+' add  '   + @columnName +  ' varchar(300) null

                                                              END'

                       exec (@SQLFieldUpdate)

                     fetch next

                     from @GetColumnName into @columnName

              end

       Close @GetColumnName

       deallocate @GetColumnName

       --exec INSERT_CERTIFICATE_FIELD

error message

Msg 207, Level 16, State 1, Line 49

Invalid column name 'CERTIFICATE_DATA'.

Msg 207, Level 16, State 1, Line 49

Invalid column name 'FIELD1'.

Msg 207, Level 16, State 1, Line 49

Invalid column name 'CERTIFICATE_DATA'.

Msg 207, Level 16, State 1, Line 49

Invalid column name 'FIELD2'.

Msg 207, Level 16, State 1, Line 49

Invalid column name 'CERTIFICATE_DATA'.

Msg 207, Level 16, State 1, Line 49