Is it permissible to perform Salatul-Istikhara for an obligatory matter, and what does the word "all" mean in the Hadith of Jabir ibn Abdullah: "The Messenger of Allah (peace and blessings be upon him) used to teach us Al-Istikhara in all matters"?

Performing obligations and abstaining from prohibitions are not subject to choice, nor is Istikhara (seeking guidance from Allah) prescribed for them, because they are binding. Istikhara is for permissible matters, to favor one over others, or for multiple recommended acts to determine the best one, such as choosing a city for seeking knowledge, or a shaykh for study, or a specific wife, or determining the date for a voluntary Hajj, and every matter involving hesitation.