The DnD 5e Dungeons Master’s Guide has guidelines for calculating the Challenge Rating of monsters that you create. Unfortunately, a DnD 5e CR calculator has never been implemented on dndbeyond. You can create monsters there, but the closest you get to a calculator is using existing monsters as a template.
I’m as big a fan of reskinning monsters as the next evil wizard. Sometimes, though, you really just want to give your players a freshly-baked monster you created from scratch. You can just wing it and fudge the numbers when things go wrong. You can also sink some time into putting together your own monster.
A Basic DnD 5e CR Caclulator
If you’re looking for a basic calculator that does all the calculations you’ll find in the DMG, I’ve found a good one for you. I’d love to give the creator more credit on this one, but they haven’t done a lot more with their website.
A Different Option
A viewer of my Twitch channel recommended this one, and it’s another good option: Monster Builder Tool | Level Up (a5e.tools). This is not the standard D&D CR calculator. It is for a system, Advanced 5e, that expands on the existing 5e rules. The assure users that their content is reverse compatible. From what I’ve seen in this calculator, that seems to be true.
But What About the Ability Scores?
The monster creation guidelines in the DMG do not give any guidance on assembling a monster’s ability scores. I did an analysis of published monster a while back, and created this Monster Ability Score Calculator. This should give you the guidance you need to create your own monster!