I agree with spending more time in the sun. Unfortunately, for many people that seems to be not sufficient.

Citing https://caloriebee.com/vitamins-supplements/Vitamin-D-Deficiency-Why-You-Need-to-Act :

Do you …

• Use a high SPF sun barrier cream
• Work long hours in an office environment
• Cover up your arms, legs, face for religious or cultural reasons
• Stay out of direct sunlight because of sensitive skin
• Have a restricted diet that excludes fish, red meat or eggs
• Stay indoors all the time through frailty or ill health (e.g. live in a care home or other institution)

If any of these apply to you, you may need to take Vitamin D tablets. You are not getting enough exposure to natural sunlight.