I don’t have this issue using 12.1 in the cloud
In[1]:= myFunc=FunctionCompile[Function[Typed[arg,"Real64"],Sum[arg^2+i, {i, 10}]]];
myFunc[18.]
Out[2]= 3295.
Perhaps it’s a bug or issue with your installation?
I do get this behaviour when using NSum—it looks like that is not compilable. Perhaps there is something you are doing with your sum that forces it to remain unevaluated or use NSum?
