For the Future
Please don't post images of errors! Rather copy&paste the actual text of the console into your question. This helps us solving the issue faster and also allows other users with the same error to find this thread.
In general
You shouldn't make huge jumps in Unity versions. 5.3.4
is from 2015, 2019.1.9
from mid of 2019 ... you can expect a lot of API changes between over 3 years apart software versions in general.
You should rather make incremental step-wise updates and also when you do be aware of what changes each version in between brings.
When using an asset from the Asset-Store that says Unity version 5.2.1 or higher
... I would suspect maintanace kind of stopped quite a while ago and there will at minimum be some severe hickups in implementing it into most latest Unity versions over 3 years later.
UNet is obsolete
This has been known for quite a while already. It will soon be (almost) completely deprecated.
The fix for now
In order to still be able to use the HLAPI for now you have to go to the PackageManager (Window →Package Manager ) and find and import the Multiplayer HLAPI
package.
See Packages for more information about the PackageManager
and packages in general.
There might still be some changes to do and I'm not saying this will completely fix that asset. If you still have trouble you should rather directly contact the makers of the asset.
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