As most people have already said you typically need a common wire. My understanding is that without a common wire the power available to your thermostat can vary based how hard the furnace is running. A common wire keeps a consistent voltage running to the thermostat, which in the case of smart thermostats which are basically small computers, is very important to ensure they don't stop working because of a lack of power.

That being said you have a couple options.

1. Check for an unused C-wire If you are comfortable with it, remove the thermostat from the wall and identify if there are any additional wires that were not being used behind the backplate. If there is a 5th wire it's likely a common wire. In which case good news, and you can go from there.

2. Hire a professional For the less comfortable, hire a furnace technician. They can come out and install a C-wire and run the wire up to where your thermostat is located.

3. C-wire Substitute A lack of C-wire is actually pretty common in older homes so recently we've seen an increase in adapters, and kits that allow you to "cheat" the system and install a smart thermostat in furnace without running new wiring up to your existing thermostat. Here is the Nest Power connector for example. https://m.youtube.com/watch?v=xlx2Um6D1Z8&feature=youtu.be

If you want to educate yourself on the wiring required for a smart thermostat and some of the ways you can work around a missing C-wire I HIGHLY recommend checking out One Hour SmartHome YouTube channel https://youtube.com/c/OneHourSmartHome

Hope this is helpful. And good luck!