Yes, there are limits on your account balance. If you have a Mettle bank account your main account balance limit is £1 million for both sole traders and limited companies.
If you have a Mettle e-money account the limits are £50k for sole traders and £100k for limited companies.
If you reach your account limit, any payments into your account would be declined and you would need to transfer money out of your Mettle account before attempting the payment again.