According to thread I linked to:
“If you still show ammonia in the pool, do the ammonia protocol, which is dose to reach 10ppm FC, wait 10 minutes, test FC again. If FC is 5 or less raise it back to 10 ppm. Once the FC is 6 or more after 10 minutes you can increase the test intervals gradually by 30 minutes as the FC begins to last longer periods of time.
If there is no ammonia test and dose the pool to reach 10 ppm of FC every 2 hours, or if you work, once before work, once after work, once at dinnertime, and once at bedtime.”
Hopefully this will zap that ammonia and then you can complete your SLAM. I know it is probably tempting to throw a boatload of chlorine in a one shot deal, but that is risking damage to your liner and equipment.