What P0740 means
Your transmission has a torque converter, and inside it is a clutch that locks the engine to the transmission directly once you're up to speed, instead of driving through fluid. This code means the computer told that clutch to lock or unlock, and something in the circuit that controls it didn't respond the way it expected. It's a broad code. It doesn't point to one bad part, it points to a circuit that isn't behaving.
Is it safe to drive
Usually yes, at least for a while. Most cars with this code still shift and drive, just less smoothly. Watch for the transmission slipping, shuddering around highway speed, harder or delayed shifts, or the engine revving a bit high for the speed you're going. If you notice the car slipping out of gear, overheating, or the transmission getting rough with any of those symptoms getting worse quickly, get it looked at sooner rather than later. Driving with a slipping torque converter clutch for a long time can wear out the transmission faster.
Common causes, most likely first
Low or dirty transmission fluid is the most common cause by far, and it's the cheapest to rule out. After that, a bad solenoid that controls the torque converter clutch is common. Wiring problems, like a corroded connector or a chafed wire, cause this code too, and those get missed if someone jumps straight to replacing parts. Less often, the problem is inside the torque converter itself, or a transmission control module issue. Which of these is likely depends on the car. Some engines and transmissions are known for solenoid problems, others more for wiring or converter wear. There isn't one answer that fits every make.
What diagnosis involves and rough cost
A proper diagnosis starts with checking fluid level and condition, then pulling live data to see how the circuit is actually behaving, not just that the code is stored. That usually means checking voltage and resistance at the solenoid and wiring before anything gets replaced. If it turns out to be low or worn-out fluid, that's a fairly inexpensive fix. A solenoid replacement costs more, and if the transmission has to come apart or the torque converter itself is at fault, that's a bigger job and a bigger bill. We can't give you a real number without looking at the car, because the cost swings a lot depending on which of these it turns out to be.
This is worth getting checked in the next week or two rather than putting off. Auto Star Service Center is open Monday through Saturday, 8am to 6pm, at 28801 Meridian E. Call (253) 375-7057 or stop by and Nick can hook it up and see what the live data is actually saying before anyone recommends a repair.