If there is an error " Function not avaibale for this responsibility" then ther might be the issue with the default.env file in Oracle Apps R12.The custom top entry should be present in the file.

 Solution:

 1. Login to middle tier with application os user.

2. Go to $INST_TOP/ora/10.1.2/forms/server  or $ORA_CONFIG_HOME/10.1.2/froms/server directory.

3. Please update  CUSTOM_TOP's details  in the default.env file.

 For Example:
 APPL_TOP=/dev/applmgr/DEV/apps/apps_st/appl
 CUSTOM_TOP=/dev/applmgr/DEV/apps/apps_st/custom

Make sure to make this changes in all the middle tier in case of multi-node environment.

4. Bounce the middle tier services.

5. Retest the issue.