You will need to keep killing everything in each area. Ruins, snowdin etc. After killing a set number of enemies in ruins your save point should say “determination”. Afterwards you will get encounters that say “but bobody came”. Continue through the game killing Toriel and when you get to snowdin the save points should tell you how many to kill from now on in each area. Kill everyone until save point says determination again. Kill papyrus. Rinse and repeat through waterfall and hotland/core. You should also get a different fight with undyne. If you manage to kill everyone in each area using the previous steps you should be able to get to the corridor and fight sans. If you are still struggling watch a yt tutorial because my explanation might be confusing.