I do enter my valid username and password but it through the error belove.

[Naga Jyothi Vasapalli]

Command: git -c diff.mnemonicprefix=false -c core.quotepath=false --no-optional-locks ls-remote https://github.com/Deva2002/Airlines.git
Output:
Error: remote: Invalid username or password.
fatal: Authentication failed for 'https://github.com/Deva2002/Airlines.git/'

[Nic Brough _Adaptavist_]

What happens if you try the git command on the command line (outside Sourcetree)?

[arbarnhart]

I am not the OP, but have exactly the same error. When I copy/paste the command into the command window it works. It asks me for user name and password and I use the exact same ones I have set up in SourceTree 3.4.2. I have removed and re-added the credentials several times. Usually, I would just type it both places but to absolutely ensure it isn't a typo I have gone through the sequence a couple of times using Google password manager as the source to copy/paste the password. Absolutely using the same username/password combo.

EDIT - It is SSH in both cases, but I have also tried HTTPS

[Patrick8639]

I have the same error. It seems that this is because GitHub does no more accepts the "old" authorization method.

Unfortunately, I don't know if it is possible to change this in SourceTree...

[arbarnhart]

I get the error with BitBucket, which makes it all the more frustrating. I know it's an open protocol, but SourceTree is more or less the "native" front end for BitBucket.