This 2020 Form 1099-R is reporting a normal distribution, apparently from a qualified retirement plan. If box 2b Taxable amount not determined is not marked, the taxable amount is shown in box 2a and box 5 would show the nontaxable amount, if any.
You apparently took a loan in 2015 and paid it back without defaulting on the loan. That essentially restored the account to what it was before the loan except with the addition of some amount of interest that you paid on the loan. Since there was no default of the loan, nothing regarding this loan would have added any nontaxable funds to the account. The interest that you paid into the plan essentially amounted to taxable investment gains; it did not add any after-tax basis.
And to answer your original question, none of this has any effect on your 2015 taxes. But perhaps you instead meant to ask what effect this has on your 2020 taxes. The taxable amount of this distribution will be on Form 1040 line 5b, adding to your AGI which is then used to calculate your taxable income. This distribution should also have been subject to a minimum tax withholding, shown in box 4 of the Form 1099-R. This tax withholding will be combined on Form 1040 line 25d to be credited against your overall tax liability in determining your refund or balance due.
