It is absolutely forbidden to ascend or enter Har HaBayit in our times. The reason is that the Beit HaMikdash was built in precise areas of extreme holiness, including the Kodesh HaKodashim and other sacred zones, where entry is strictly prohibited without the proper state of ritual purity. Since today we are all considered tamei ritually impure. It is impossible to determine with certainty which areas are permitted and which are forbidden.