这是因为你没有将合约和网络绑定或者ABI使用错误。正确代码如下:

from tronpy import Tron,Contract
import json
client = Tron()
cntr = client.get_contract('TGjYzgCyPobsNS9n6WcbdLVR9dH7mWqFx7')
cntr.abi=json.loads(mystr)
jUSDC = "TNSBA6KvSvMoTqQcEgpVK7VhHT3z7wifxy"
print(cntr.functions.markets(jUSDC))

mystr='''
[
{
"constant": true,
"inputs": [
{
"internalType": "address",
"name": "",
"type": "address"
}
],
"name": "markets",
"outputs": [
{
"internalType": "bool",
"name": "isListed",
"type": "bool"
},
{
"internalType": "uint256",
"name": "collateralFactorMantissa",
"type": "uint256"
},
{
"internalType": "bool",
"name": "isComped",
"type": "bool"
}
],
"payable": false,
"stateMutability": "view",
"type": "function"
}
]
'''