Hi, I recently purchased a booster Heater for our church camp dishwasher. It came with 208/ 3ph connections and said it could be field converted to 208 single phase. I checked the voltage at the panel and it read 140+ volts. Is this a problem?

Since it's a church camp you probably have a single phase service. On a single phase service you would only have 120/240 volt power. Each incoming hot line to neutral should be around 120volts and reading from hot line to hot line should be around 240volts. If your booster heater is only capable to be wired for 208 volt single phase you may want to exchange it for a booster heater capable to be wired to 240volt power. Read the manual that came with the heater and see if it is capable to have 240 volts,some componets in the heater may require that it is only good for 208 volt power.

Is "140 volts" a typo mistake?

Yes, is "140 volts" a typo? But either way, would you please
Measure each hot leg or phase conductor to ground and then measure
The voltage hot leg to hot leg...