Systems from Acer, Apple, Dell, HP, Lenovo or other major brands often use motherboards which differ slightly from the specifications issued by Intel and AMD. While those changes are usually subtle, they can lead to compatibility issues with coolers that were built to comply with these specifications.
Even in case the cooler is mechanically compatible and can be installed, other issues can occur, e.g. proprietary fan connectors, BIOS errors due to a low fan speed, shutdowns, etc. Some of these problems can be avoided with some technical knowledge, but especially BIOS related issues often cannot be resolved.
Due to the large number of possible issues that cannot be resolved with different mounting parts alone, Noctua does not officially support systems from Acer, Apple, Dell, HP, Lenovo or other major brands.