Mix the cola, brown sugar, garlic powder, onion powder, and salt in the insert set in a 5-, 6-, or 8-quart Instant Pot. Add the pork and stir well to coat. Lock the lid onto the cooker.
For the MAX model:PRESSURE COOK on MAX. Set the time for 1 hour with the Keep Warm setting OFF. If needed press START.
For other Instant Pot models: Set for PRESSURE COOK on HIGH pressure. Pressure valve must be closed. Set the time for 1 hour 10 minutes with the Keep Warm setting OFF. If needed press START.
When the pot has finished cooking, turn it off and let the pressure return to normal naturally, about 35 minutes. Unlatch the lid and open the cooker.
Use a slotted spoon to transfer the meat to a nearby large bowl. Use a flatware tablespoon to skim the sauce in the pot of its surface fat.
Press SAUTÉ and set it for MEDIUM, NORMAL, OR CUSTOM 300°. Set the time for 20 minutes. If needed, press START.
Bring the sauce to a simmer and cook, stirring occasionally, until it’s reduced to half its volume from when you removed the pork, about 10 minutes.
Stir in the enchilada sauce and continue cooking, stirring more frequently, until the sauce has again reduced by half its volume after you added the enchilada sauce, about 8 minutes.
Pour the pork and any juices in its bowl into the insert. Stir well and cook, stirring often, for 1 minute. Turn off the SAUTÉ function and cool for a couple of minutes. To serve, use kitchen tongs to pick out some of the meat from the sauce. Set the meat on top of a tostada shell, then top with shredded iceberg lettuce, black beans, crumbled queso fresco, and a lime wedge for squeezing over the whole thing.