Pull the drive out - it's a fairly tight fit. At the bottom of the hole are two rubber segments where the drive is usually seated. Unfortunately these are custom shaped to match the original hard drive shape exactly - and won't seat the new replacement drive going in which has a chunkier profile.

The two rubber segments have to come out for trimming, and then placed back inside which is all quite fiddly but doable using a long thin wooden implement, such as a chopstick, to fish them out, and then prod them back into place.

Confirm that the trimmed segments are both located back in the correct position at the bottom of the drive bay before proceeding.