If the ice and water shield is missing then you will have to remove the shingles on both sides along the valley put ice and water shield in place and re shingle the roof.
How to repair a roof valley leak.
Follow these three preparatory steps to get ready to fix the valley leak.
Many roof valley leaks are caused by roofing nails that have been poorly.
How to repair a lead valley.
Clear any debris in the roof valley and surrounding area.
Move to the highest end of the valley and work your way down cleaning.
Step 2 remove the shingles.
A properly flashed roof valley should have an ice and water shield installed with roofing felt being properly secured over the ice and water.
Step 3 lay the flashing.
And you can tar it but it s a real stop gap band aid like measure.
The right thing to do here is to pull a valley basically pull a valley apart and rebuild it.
Find the place where that leak is coming from.
Step 4 replace the shingles.