IF your code are correct, you may fix up the problem with 'aseembly permmistion_set'

CREATE ASSEMBLY assembly_name

from

WITH PERMISSION_SET = { SAFE | EXTERNAL_ACCESS | UNSAFE }

--1. using function of object_id

if object_id('object_name') is not null begin

print 'Exist'

end else begin

print 'Not exist'

end

--2. using sys.objects & exists function

if exists(select...