MySQL错误1822：添加外键约束失败;约束缺失索引BUT索引存在

I am trying to add an foreign key to my flightschedule table but it fails, but I do not really know why. The foreign key should reference the txtAC_tag attribute from the tblAircraft table which is part of the Primary key! So the tblAircraft is indexed (the primary key is a combined key which consists of idAC and txtAC_tag -> could a combined Primary key be the problem?) and the attribute's datatype do match.I am trying to add an foreign key to my flights