Nick Milas writes:
dn: uid=userx,ou=people,dc=example,dc=com changetype: modify add: ObjectClass objectClass: posixAccount uidNumber: 1700 (...)
With 'changetype: modify', for every attribute type you need 'add: <attrtype>' in front and a line with just '-' after, to separate it from next attr.type. See 1st 'man ldapmodify' example.
Beyond that - it's usually easier to help when you give the error message and the command line you used, not just "it fails".