if it was a bad private key, you would get the error when trying to import it. and blockchain.info wouldnt be able to tell you the balance as it wont be able to read the private key to translate the public key.

so if its showing a balance but not letting you spend it.. then its nothing wrong with the private key. but instead how blockchain.info is coded.

i say this because your screenshot appears to be on the transaction page not the import private key page. so based on that i feel it is a fault on blockchain's part.

if however it is related to an error appearing when you import the key.. check you didnt copy it, but accidentally including a space before or after what you highlighted to copy, i have seen a few online services error out due to a simple space typed in before or after the key

last tip:
if anyone is personal messaging you to tell you they are an expert and can help, all they need is the private key you used for them to check it.
or if they tell you a website to go to to check it that you have not heard of before... DONT give it.

the other option is just to use a different and proper bitcoin wallet.
remember the funds are not locked into blockchain.info account.. they are locked into the private key. so use the private key on a proper wallet