问下如下报错是什么原因，谢谢
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问下这个报错信息是什么情况以及如何解决，谢谢Node set assembly_part-2-1_part-2-refpt_ has not been defined
Unable to locate node set assembly_part-2-1_part-2-refpt_. The node set must have been previously defined.
Node set assembly_part-2-1_part-2-refpt_ has not been defined
Unable to locate node set assembly_part-2-1_part-2-refpt_. The node set must have been previously defined.
Node set assembly_part-3-1_part-3-refpt_ has not been defined
Unable to locate node set assembly_part-3-1_part-3-refpt_. The node set must have been previously defined.
2604 elements have missing property definitions. The elements have been identified in element set ErrElemMissingSection.
Analysis Input File Processor exited with an error.
Unable to locate node set assembly_part-2-1_part-2-refpt_. The node set must have been previously defined.
Node set assembly_part-2-1_part-2-refpt_ has not been defined
Unable to locate node set assembly_part-2-1_part-2-refpt_. The node set must have been previously defined.
Node set assembly_part-3-1_part-3-refpt_ has not been defined
Unable to locate node set assembly_part-3-1_part-3-refpt_. The node set must have been previously defined.
2604 elements have missing property definitions. The elements have been identified in element set ErrElemMissingSection.
Analysis Input File Processor exited with an error.
