Meta can’t always accurately match 100% of events or clicks to a targetable Facebook or Instagram user for building a custom audience. This can be for a variety of reasons but usually, it is due to the iOS14 privacy update.
You can still try to create an LLA using this audience and then use both the Custom Audience and LLA in your ads.
The initial size of your Custom Audiences and LLAs may not be large enough to be effective at first but as you run ads for longer your Custom Audience size will grow which will automatically update the associated LLA.
Just be sure to add all of your Custom Audience and LLAs into 1 separate ad set and make sure you run it against other interest-based ad sets so that Meta still has a large enough audience size across your campaign to remain effective.