A purchase pool is the remaining sum of a coin, you can add to it with a purchase and reduce it by a sale.
Generally speaking, every coin in the system needs a cost basis assigned to it as a basis on which to calculate gains/losses when this coin is sold again.
Such a cost basis is established (most commonly) by a trade, but also by the transaction type income/staking/mining/gift/airdrop/ income (non taxable).
A cost basis is NOT generate by a deposit. A deposit is just the transfer of an existing coin within your own wallets or exchanges and needs a matching withdrawal.
The purchase pool warnings "There is no suitable purchase to this sale (all purchasing pools consumed) are most often caused by the fact that there is only a deposit of a coin, but no cost basis inducing transaction before. It is as if this coin gets dropped out of thin air without a cost basis - and when this coin is finally spent, a warning appears.
So the warning in the tax report might be at a much later date than the coin came into the system (without a cost basis) and you have to look through the coin's history to find the origin. In this case the Missing Transations Report will give helpful hints for questionable single deposits as white lines: White entries (no match) in Missing Transactions Report
In addition you need to consider that the tax report uses the coins with a certain method and in a certain order. So even if you bought BTC for FIAT on an exchange (=cost basis established) and then immediately sell it for another, the BTC used to calculate the gains for that sale might be from a different purchase date and could thus have no cost basis.
It is possible that direct debit and credit card purchases are imported just as deposits as well. In this case please change the deposit into the underlying trade of FIAT into Crypto that was skipped.
In the special case that you are using depot separation, it is possible that a there is a warning / a coin has no cost base, because the correct sequence of withdrawal first, deposit second was not kept. You can check for this in the transaction flow displayed under the depot separation option in the tax report.
In a nutshell & how to move ahead:
In this case transactions are missing or wrongly added. Please make sure you have all transactions added correctly as described in those FAQs - now with some videos as well - as all reports/balances are calculated based on the transactions you did import:
Adding all transactions from your transaction history across all your used exchanges/wallets is mandatory to get consistent balances and reports.
All balances at any time are calculated based on the transactions you have in your account.