My bank account is linked and I can sell but cannot buy?
Any ideas...
Does your bank have a block on crypto related transactions?
If you have enough funds in your bank, and you are sure that your bank account is linked properly (being able to withdraw, I think chances are that it is), and it still doesn't work, it's probably something on the bank's side.
Don't just contact Coinbase support only - also contact whatever bank you are with. Make sure you are not too explicit about the transaction being crypto related though, since you could alert them to something that they didn't even know of before and flag your account subsequently.