The scriptures do not allow worship of demigods, but can I worship Lord Nrisimha who is an incarnation of Lord Vishnu for material desires. It is also said that by reading the Nrisimha Kavacha all the material desires are fulfilled. Is that true?
The scriptures explain that whether we have no material desires or are full of material desires one should worship Krishna only. By worshipping the demigods for satisfying material desires one gets more conditioned in this material world whereas by worshipping Krishna one gets freed.
By regularly chanting the Nrisimha Kavacha or any other prayer that glorifies Vishnu, one may be able to fulfill material desires, however the only result that we can expect in the long term is pure unalloyed devotional service (Bhakti).