They could not enter in because of unbelief.
To allow unbelief is to block your access to greatness.
Unbelief cuts short destiny. Hebrews 4:1
Unbelief destroys profit. Hebrews 4:2
Unbelief robs people of rest. Hebrews 4:3
There remaineth therefore a rest to the people of God. Hebrews 4:9
We are not called to work; we are called to simply believe.
What shall we do that we might work the works of God? This is the work of God, that ye believe on Him whom He hath sent. John 6:28, 29.
The only labor we have to engage is faith.
Let us labour therefore to enter into the rest. Hebrews 4:11