请问各位大佬  我用ivregress 2sls 做工具变量的时候，为什么工具变量因为共线性被剔除了 工具变量是美国EPU，内生变量是中国EPU  被解释变量为基金净值增长率
ivregress 2sls GNR MRR NRR logSOC logSOF CPI logDAYs (logCNEPU=L.logAMEPU) i.quarter i.fundid,r
note: 220.quarter identifies no observations in the sample
note: 248.quarter omitted because of collinearity
note: 249.quarter omitted because of collinearity
note: L.logAMEPU dropped due to collinearity
equation not identified; must have at least as many instruments not in
the regression as there are instrumented variables
r(481);