With very little information, it is hard to say what is the cause. Please refer the example: https://optics.ansys.com/hc/en-us/articles/1500006203341-GDSII-Export-Automation
I would suggest to test the case with fewer geometry and see if it works. GDSII supports limited number of points, depending on the version. Here are a few posts that might be helpful:
Lumerical FDTD gds export vertices limit
如何导出结构组？
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