Make sure all you test information is correct and matches your passport as far as name goes.
We did have one guy get denied boarding at his home airport because his test didn't have the word "Nasal" printed on it even though the test listed was a nasal swab and could be confirmed by a quick google search. This was at his home airport in the US.
It took a lot of hassle on his part to get that straightened out before being allowed to fly to MIA. He got his test through one of those Any Test Now, diagnostic clinics.
United pre-approved everything in their mobile app. Nothing was checked at the airport. T&C is a perfunctory check of your docs before Immigration. Complete joke.