In Identity Manager 12.6.7 and lower, the maximum length an attribute can have is 36. This error will show up:
"Global User 'TestUser' creation failed: The length for attribute 'Global User Name' is greater than the maximum of 36 ]"
To avoid this issue, please upgrade to IM 12.6 SP8 or later. The length is extended to 132. This is set in the cosparse.ptt table in provisioning server (...CA\Identity Manager\Provisioning Server\data) and can not be modified.