A.T. Robertson Commentary Hebrews 4:6

"Seeing therefore it remaineth that some should enter thereinto, and they to whom the good tidings were before preached failed to enter in because of disobedience," — Hebrews 4:6 (ASV)

It remaineth (απολειπετα). Present passive indicative of απολειπω, old verb to leave behind, to remain over. So again in 4:9; 10:26. Here the infinitive clause (τινας εισελθειν εις αυτην) is the subject of απολειπετα. This left-over promise is not repeated, though not utilized by the Israelites under Moses nor in the highest sense by Joshua and David.

Failed to enter in (ουκ εισηλθον). "Did not enter in" (second aorist active indicative of εισερχομα). It is a rabbinical argument all along here, but the author is writing to Jews.