Description:
The error is caused by Synplify Pro generating too many GB (Global Buffer).
Solution:
There are 2 solutions that will work as stated below:
1. The solution is to set the attribute below in your RTL:
/* synthesis syn_global_buffers = */;
This is better explained in Synplify Pro's help file.
2. The other way is to demote a GB in iCEcube2 Floor Planner view. (global tab at the left pane.)