This sentence. From what I can glean from your question, your confusion stems from "feeling a lack", with a lack being a void or negation or absence. How can one feel an absence?

However, one can feel (or perceive) a lack. For example, "feeling cold" means to feel a lack of heat, in the same way that "feeling hot" means to feel an excess of heat.

Or to use an example from your sentence, "feeling lonely" is also feeling a lack, specifically a lack of companionship or community.

If you would like to rewrite the sentence, I would suggest:

After returement, some people feel lonely and without a purpose in life.

although KeykoYume's suggestion would also work.

This sentence is valid. From what I can glean from your question, your confusion stems from "feeling a lack", with a lack being a void or negation or absence. How can one feel an absence?

However, one can feel (or perceive) a lack. For example, "feeling cold" means to feel a lack of heat, in the same way that "feeling hot" means to feel an excess of heat.

Or to use an example from your sentence, "feeling lonely" is also feeling a lack, specifically a lack of companionship or community.

If you would like to rewrite the sentence, I would suggest:

After returement, some people feel lonely and without a purpose in life.

although KeykoYume's suggestion would also work.