ERROR:Pack:1107 - Unable to combine the following symbols into a single IOB

大家好，最近我在写一个数据储存的VHDL代码，当我再给一个时钟管脚分配位置时，出现如下错误，弄了好半天也没弄明白，希望路过的高手能指点指点，谢谢啦！！！
ERROR:Pack:1107 - Unable to combine the following symbols into a single IOB
   component:
    PAD symbol "fosc_j" (Pad Signal = fosc_j)
    BUF symbol "fosc_j_IBUF" (Output Signal = fosc_j_IBUF)
   Each of the following constraints specifies an illegal physical site for a
   component of type IOB:
    Symbol "fosc_j" (LOC=P15)
   Please correct the constraints accordingly.