AFTER UPDATE trigger that modifies the table on which it is defined
You don't need, neither you can have (as the error message suggests), the UPDATE statement against the table on which the trigger fires. You can, however, directly set the value any NEW column in a BEFORE trigger:

CREATE TRIGGER `Metrosky_spotters_2020_au` 
BEFORE UPDATE ON `Metrosky_spotters_2020` FOR EACH ROW
SET NEW.trnmswid = 5000