Do you know if its your address you see in the addresses tab?
Yes, in my old wallet I have two receiving addresses and and in Electrum I see one of them (starts with "1PV"....)
I'm thinking it's likely you've imported the wrong type of address.
According to the link I'll put at the bottom of the screen you might want to add lines of:
p2pkh:private key
p2wpkh-p2sh:private key
p2wpkh:private key
Yes, I have "p2pkh:........." because guidelines in "info" button says "p2pkh:K...... -> 1......" so first characters of key and address are correct.
1. Did you check your address on a block explorer for funds (chainflyer.bitflyer.jp ; blockchain.com ; ...)
On my old wallet app (where I imported private key from) I see funds and history
2. Is the address shown by Electrum the same as the one you have funds on?
Yes
3. At the step where you pasted the private key, did you click the "info" button and followed the guidelines?
Yes, I wrote there "p2pkh:[my_priv_key]" because guidelines in "info" button says "p2pkh:K...... -> 1......"
Another thing I realized is that I run Electrum on fresh Ubuntu 20.04 LTS install on VirtualBox VM. Network connection is set to NAT and I have internet connection on that VM and Electrum server status is green, but maybe it's related to port forwarding or internet connection configuration on VM ? I know I can check installing it on my host computer but unfortunately I don't trust my host windows machine enough to send my private key there.