General Description: There are several reason why this error is generated. Here
are a few examples:
1- In the Global Tab, the Clock Net Names are machine generated names,
$Net00012. When CE creates a Period constraint for that net, it will name the Time
Spec Name TS_$Net00012. CE does not allow the use of $ in Time Spec Names.
2- In the Advanced Tab, when creating FROM:TO constraints, CE does not allow the
use of special characters in the Time Spec Name. i.e. !, @, #, $, %, ^, &, *, ..
The resolution to this is simply remove the special characters in the Time Spec
Name. CE will then create the constraint.