Here is a better explanation on “why can’t I send transactions to CoinGate invoice from an exchange?”
Below is what CoinGate explained on its blog;
We do not recommend sending transactions from cryptocurrencies exchanges/web wallets, because transactions from exchanges get sent after a significant delay and sometimes the exchange charges a fee from the sent amount. As a result, transactions often arrive too late (when the invoice has already expired) and / or not enough amount is sent (so the invoice is underpaid). This is why we recommend using one of many opensource wallets, where the sender controls the exact amount sent and when the transaction is sent.
Furthermore, you do not control your own bitcoins when using a web wallet, so you introduce counterparty risk and thus negate one of the main benefits of cryptocurrencies – to be in full control of your own money.
Please read our blog post on how to install and configure a cryptocurrency wallet here.