The End

Endportal is build wrong. The Endportalframes have a direction that is very important. The Frames should face into the portal.

I can see in your build that the frames are all facing in the same direction (look at the stripe that is going right through them). Try to stand in the middle and build the frames around you and it should work.

I had the same issue. Try breaking and replacing some portal frames, it works eventually.

This post helped me.