I need to sign my 2019 tax return to apply for a student loan.

When you e-file a tax return it is signed electronically when you enter the correct AGI from the previous year's tax return. There is no physical "signature" on the page.   When you e-file it you get back an email showing that the return was accepted, and you can print out the page that shows the filing "postmark" when you print the return.  You are way too late to e-file a 2019 tax return--e-filing closed in October.  The only way to file a 2019 tax return now is to print it, sign it and mail it.

If you already filed it and now you just want a "signed" copy for FAFSA, etc., take out a pen and look for the line that says "Sign here" on your Form 1040 and sign it.