4
I think, the code-based sandbox solutions are no longer going to work in SP 2019. It's one of the most promitent things removed in the latest sever version of the product:
After careful consideration, we've decided to also remove support for
code-based sandbox solutions in SharePoint Server 2019. Customers are
recommended to explore SharePoint add-ins as an ...
2
I agree with Denis, as mentioned this feature has already been removed in SP 2019.
1
Try this
<script src="../../jslib/jquery351/jquery351.min.js"></script>
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