[Ethereum] Web3.py Could not transact with/call contract function, is contract deployed correctly and chain synced

is contract deployed correctly and chain synced?

That error message is right. You should definitely sync fully, and that will apply to the main net, too. You might be pleasantly surprised how much faster parity syncs.

unknown account

This problem is unrelated to syncing. It also sounds like you are trying to transact from an account that geth doesn't have a key for. web3.eth.accounts needs to show the account you set as the from field in transact().

I cannot assign more gas above gas limit of ganache... is it possible to increase this limit.... ?

Yes, start Ganache with --gasLimit=someHigherValue.

That being said, note that 6721975 is already by itself a pretty high value for a single transaction, so your deployment most likely fails for a different reason ("out of gas" is unfortunately quite a misleading error-message, but I think that Web3.js dev team are aware of that and are working towards fixing it in a future release).

In order to check whether or not your gas value is the problem here, replace this:

send({from: account1, gas: 6721975, gasPrice: '30000000');

With this:

estimateGas({from: account1);

And then make sure that it completes successfully.

If it does, then your gas value is indeed the problem, and you should replace it with whatever the call to estimateGas has returned.

Otherwise, your transaction reverts for a different reason, most likely related to the contract function itself (i.e., your contract's constructor).

One more thing to notice (which I recently encountered myself and took a lot of effort in figuring out):

Ethereum has a contract code-size limit of 24KB.

If you contract's code-size exceeds this limit, then no matter what you do - whether it's in your contract code or in your deployment code - the contract deployment will fail.

How to check your contract's code-size:

1. If you're compiling via solc executable, then make sure that the size of the output bin file does not exceed 48KB
2. If you're compiling via solc.js (for example, via Truffle), then make sure that the size of the bytecode field in the output json file does not exceed 48KB

Why 48KB?

2 hexdecimal characters = 2 bytes in the text file = 1 byte in the actual bytecode.