On possible reason (although I saw this on older versions...not 7.1):
Check your vsource statements. Do they all have a dc value set?
If they do not, you will see this error.
If you put a dc value on the vsource, the error will go away.
I recommend upgrading to MMSIM 7.2 (latest ISR), as this issue may go away with a newer version of spectre. Otherwise, please contact Cadence Customer Support.
Staff Support AE, Global Customer Support
Cadence Design Systems, Inc.