Myanmar does have SY-400 system.
However, the SY-400 system is not a missile, but a long-range rocket system that is accurately positioned by China's Beidou satellite system and then strikes. It is similar to the HIMARS rocket system in the USA, which uses GPS to locate and then carries out strikes. Therefore, the Myanmar military govt may not be able to continue to use SY-400.
Because the SY-400 was ordered by Aung San Suu Kyi's govt from China in 2016 and delivered to Myanmar in 2020. But in 2021, Aung San Suu Kyi was subjected to a coup by the Myanmar military govt.
Subsequently, the Myanmar military govt threatened China to recognize its legitimacy by suspending the B&R Project. Relations between China and Myanmar's military junta have also deteriorated.
These SY-400 may not be able to be used. Because China can cut off the Beidou satellite system. If SY-400 lacks the intelligence support provided by the Beidou system, it will be a pile of scrap iron.
The Burmese military govt has suspended China's B&R projects, and they also want to coerce the Chinese govt into making political concessions. The relations between the two countries are at a low ebb. Before the return of Aung San Suu Kyi's govt, the relations between the two countries cannot be restored to the previous state.