i don't know hoe heavy your 'low-poly' count is, but more likely your convert settings need to be adjusted.

check the script editor what it says.

if there is a error like:
Error: polyToSubdiv1 (Poly To Subdiv Node): Cannot create subdivision surface as the resulting surface would have more base mesh faces (14400) than the maximum allowed (1500).


Go to modify->convert->poly to subd->option box paste the first number, in this case 14400 into the maximum base mesh faces.

That should do it.