They failed to pay, a letter was sent, then they sent a payment minus all owed interest, now it's past the grace period. They had to pay a certain way which all parties originally agreed to but because they want to conserve certain funds they tried to modify the terms unilaterally. At this point they still have the funds but don't want to use up the hard currency they still have.

It's my understanding that Russia is ready and willing to pay its debt but has been forced against its will to default by US government sanctions. Russia even anticipated this and payed 2 payments in advance[1].

[1] https://news.yahoo.com/us-end-russias-ability-pay-192110081....