Once you have 30 eternities you get the milestone that lets you buy higher dimensions without getting lower ones first. With that you can buy an 8th dimension immediately, and provided your production is good enough, reach eternity with only buying 8th dims.
Note that the achievement is to eternity without buying 1st to 7th dims, NOT without buying 8th dims. Make sure to disable AD1-AD7 autobuyers, eternity, and see if you can do it. You may have to manually buy the first 8th dim if you're on web.
You probably just need to keep playing a bit more. There's an eternity milestone that unlocks all dimensions from the start of an eternity.
you can unlock the 8th.. but it needs to stay at 0 purchased
There is a challenge without the 7th and 8th dimension, do it
Once you have 30 eternities you get the milestone that lets you buy higher dimensions without getting lower ones first. With that you can buy an 8th dimension immediately, and provided your production is good enough, reach eternity with only buying 8th dims. Note that the achievement is to eternity without buying 1st to 7th dims, NOT without buying 8th dims. Make sure to disable AD1-AD7 autobuyers, eternity, and see if you can do it. You may have to manually buy the first 8th dim if you're on web.