I struggle to meet my calorie goal. Is it bad?

if you are eating 800-900 calories daily yes that is quite bad and can cause long term health issues.

but i do recommend getting a food scale to better know what you are indeed eating. 1200 is the bare min a verrry short adult female who barely moves should eat for proper nutrition.

https://community.myfitnesspal.com/en/discussion/10569458/why-eating-too-little-calories-is-a-bad-idea/p1

Weigh that handful of nuts. They may surprise you.

How long have you been doing this and how have your losses been (and what are your stats and goal)?

You could be underestimating -- weighing everything would be a good idea -- but that doesn't sound like a lot of food. My guess is that it's more a "just starting out want to eat as little as possible" thing that will take care of itself, but it doesn't seem that healthy.

My estimate -- breakfast would be 140-210 cal of eggs (assuming not cooked in any butter or oil), about 300 cal of oats (with only water, I assume), plus 50-100 (a little bit with high cal items can be way off) in butter or oil, so about 560.

Apple would be about 100 cal, nuts or seeds (.25 cup/one oz) would be 160+ cal, but could easily be more, I'll say 200 and average for 150. So 710.

Dinner, depends on how much meat you eat, whether there's any oil on the veg, etc. But my guess is the total is more than you are estimating.

That said, I think your "clean" approach is too much about what you aren't eating than what you are. The apple (or other fruit!) shouldn't be only if I'm hungry, but could be included in the breakfast, as could foods like seeds and nuts (in the oats) or nutbutter, or some avocado. It doesn't add many cals, but I'd eat way more than 1 cup of veg a day, and you could include that with the eggs. So on. For dinner, good to choose higher fat chicken if no other fat, but it would be easy to bump up the cals, nothing wrong with including a nutritious starchy side like potato or sweet potato or a whole grain, or some fat with the veg, and I am wondering about total protein -- probably not nearly high enough if you are eating even close to the cals you assume.

Personally, one reason I eat 3 meals is that I couldn't get in the all veg and protein I think are important on just 2 (this isn't everyone, but could be you).

I agree with the others to get a food scale to help you measure more effectively than measuring cups. But if you are in fact eating 800-900 calories a day, yes that is too low, and doing so over a sustained period of time could lead to negative health effects.

Which of you experts have a degree in nutrition science? May I suggest you visit with a doctor who will assess your health status and visit with a Registered Dietitian who will assess your macro and micro nutrient needs and assist you in creating a plan for healthy, mindful eating!
Not eating puts your body into starvation mode!
Your body needs approximately 60 different nutrients, provided in a wide range of foods consumed in varing amounts. Without these nutrients your body can not grow, repair or nourish cells, tissue and organs. The same is true for your children and grandchildren. Perhaps it's time to take a nutrition class from your local junior college -- on line will meet this need.