WIF keys are the only ones you should use. The uncompressed (begins with 5) and compressed private keys (K or L) correspond to different addresses. Look them up on a block explorer to find out if any coins were sent there and then import the corresponding priv key.
WIF keys have a checksum in them that detects typos. The reason you couldn't import the uncompressed priv key is because you made a mistake copying it or copied extraneous characters. That's why the next button was greyed out.
much agree with Abdussamad's explanation and advise
since using compressed private key doesn't seem to import the correct address,
OP should definitely retry importing the uncompressed private key
if it still doesn't import the correct address, then try nc50lc suggestion
@OP you do know your actual bitcoin address, right?