Function Not Available To This Responsibility - Custom Form

Symptoms :
  When user open the custom form in oracle application, error "Function Not Available To This Responsibility" appears.

Cause :
  Path of $CUSTOM_TOP has not been registered in the default.env file.

Solutions :
1. Login to server with APPLMGR manager user access.
2. Go to $INST_TOP/ora/10.1.2/forms/server directory.
3. Ensure that your CUSTOM_TOP's are registered in the default.env file. This file present  under location $INST_TOP/ora/10.1.2/forms/server directory. 
4. If has not been registered, you should create an entry for environment variable CUSTOM_TOP (which contains physical path to your custom directory). 
For example: 
...
ZX_TOP=/u01/PROD/apps/apps_st/appl/zx/12.0.0
XX_TOP=/u01/PROD/apps/apps_st/appl/xx/12.0.0
5. Save that file and re-run your custom form