I don't find a status ISSU in the SAP table TJ02T in my system, TJ02T is the table for SAP's system statuses where ESTO could be found.

As you said user status, your question is actually like this: How can I change the status from system status ESTO into user status ISSU?

Since system status and user status are kept in different fields in an equipment I think this is a kind of impossible.

Nobody here knows your system and hence cannot know what ISSU means so you would need to explain what the status is, we could imagine it means "goods issued" , and based on this assumption we could answer with: do a goods issue. But better than speculations would be to ask the colleagues who defined the user status in your system.