In versions prior to 2019.11 the Assert-Methods, did not belong to a namespace.
Assert_Equal_INT(expected:=8, actual:=res);
Since 2019.11 the Assert-Methods belong to the AxUnit.Assert namespace and use overloading to make their use more convenient. Please make sure, that you exchange your AxUnit.st file with the new Version of this release. Therefore download the SDK and navigate to AxUnit.Library folder, copy and paste the contents from the st file to the AxUnit.st in your project. Finally, adapt the Assert-Functions of your test code according to the new conventions:
Axunit.Assert.Equal(expected:=8, actual:=res);