A coin can be used to stake once it has reached coin maturity. When you send STRAT to your staking wallet, you need to wait until the transaction has received enough confirmations before the STRAT can participate in staking. This means that the transaction needs to be included in a certain number of blocks before the network will recognise its contribution to your network weight. Similarly, a newly staked STRAT will need to mature before it can contribute to staking.
For STRAT, coin maturity comes after 500 confirmations. At a block time of around 64 seconds, that means a maturing time of roughly 8hrs 45 minutes.