At it's most basic level strength training is a relatively simple process. You pick a handful of movements that incorporate all of the musculature of the body and execute those movements on a weekly basis. You drive favorable adaptations by adding load (weight), repetitions, sets, altering tempo, or cutting down rest intervals over time. For newer trainees, younger individuals (under the age of say 60ish), injury free, and illness free people this process is so simple that you almost can't screw it up. Common mistakes are overloading past recovery abilities, or injury resulting from high risk movements.
Where things get complicated is when injuries, illnesses (particularly chronic issues like panic/anxiety disorders and auto immune issues), and older ages (say 70+ in most cases) are thrown into the mix. Many people in these situations simply avoid training or do to little to actually see improvements. What I've seen with my own clients over the last couple of decades is that they still benefit from progressions, it just has to be at a much slower pace. Sometimes their progressions aren't linear. On certain days, we may end up taking 2 steps forward and 1 step back. Dealing with clients who struggle with auto immune and anxiety related orthopedic issues can be the most challenging. Stress in general can manifest as musculoskeletal pain (particularly in the upper traps, neck, shoulders, and sometimes hips and lower back). We need the stress of exercise to spur posotive adaptations, but in these populations too much stress can aggravate an injury, outpace their recovery ability, or set off a painful somatic response.
A few things that I've found to be helpful in these demographics:
1) Gradually building up pain free range of motion (if mobility is restricted due to discomfort).
2) Taking smaller jumps in load, sets, or reps. While a typical client is fine increasing one or multiple variables every training session, these demographics often need 2-3 sessions of identical workouts before progressing. When progressions are made, I've found it can be helpful to only increase one variable on one movement at a time. This makes for much slower progress, but it's still progress, and it builds confidence. This is especially helpful in injured populations, or individuals who struggle with autoimmune or panic/anxiety issues who often associate pain and discomfort with exercise.
3) Reducing training volume, and keeping intensity slightly lower. The average client can handle 3-5 sets of most movements, and can get close to failure, and recover just fine (assuming technique is maintained). These other populations seem to do better with 1-3 sets, and staying a bit further away from failure.
The big takeaway here is that nobody is unable to do fitness training! With the right modifications, and programming virtually everyone can see significant benefits from a solid strength training program. The increases in confidence, sense of accomplishment, and improvements in capacity of activities of daily living and sport all work towards increased quality of life. If you struggle with pain/fear in regards to strength training I encourage you to find a qualified coach, and communicate clearly while working with them. You'll find that the benefits are absolutely worth it.