1. Check the Private Key Format:
Please read the entire thread first.
And he already answered that in this quote:
Yes it does start with 5
-snip-
4. Choose "Use a master key".
5. Instead of a seed phrase, paste your WIF private key (starting with 5).
This is wrong, uncompressed WIF private key will not be accepted in that menu.
Electrum will NOT sweep uncompressed keys but will accept them in wallet creation.
Wrong again, it accepts uncompressed and compressed WIF in both Import (
wallet creation) and Sweep.
Method 3: Bitcoin Core (Slow but Sure)
You can install Bitcoin Core, let it fully sync (can take days), then:
1. Run the console (under Tools).
2. Run:
importprivkey "YourWIFKey"
You should use an alternative with an updated database since this is for older versions of Bitcoin Core
or at least cross-check the results first
before posting it as a reply.