Can I plan for future while doing meditation on death?

meditation on death is not what on dhamma. It's a misunderstanding by some people. It’s mentioned to focus on rising (udayangama) and falling(=death) (aththangama) of five aggregates which are rupa, vedana, sagna, sankara, vingnana. Instead of this now some teachers are teaching to focus only on falling or death or ending of the life but not about aggregates.

So find the correct teacher or study the canon by your self. But translations has many mis interpretations due to the lack of knowledge in original language.

Buddha's advice was to always concentrate on the present moment, but it may prove very challenging for basically for any regular person. Planning about the future cannot be stopped if you are living as a layperson therefore I don't think you should stop doing that either. But as Buddha said if you could at least think that you will die every day (at the beginning) you could have less attachments in life. Later on you can increase the frequency to every breath but it may prove difficult for a layperson as it is much more suitable for a monk who has less duties and bondages.

Another way of doing the meditation on death (Taught to me by a Thero) would be to think how you would be on your deathbed, you will lose your senses, lifting your arms or even your fingers would be an almost impossible task. Regrets in life on not being able to cultivate Kusala Kamma (Kusal is different from merits, as Kusal leads you to liberation while merits lead you to more births which are comfortable) etc... That way you would be able to become much more motivated to achieve Nibbana as soon as possible.

Recollection and Mindfulness of the death is a meditation for people who have weak willpower to acheieve nibbana and it could also be used for worldy achievments as well but is not recommended.

Insight meditation generates good karma (Kusal), as when you see the truth (past the veil of illusion) you see what it is for what it is, which is nothing. When recollection and Mindfulness of death is done to a certain point you will realise that we are dying and being born every second but this would be a bit complex for now, but I digress. When meditating on worldly things like "(thinking about failure of an ongoing project, end of a relationship etc.)" it doesn't generate Kusala Kamma as it is not real (Again the concept of instant death, which leads to Impermenance (Anicca) and Soullessness (Anatma). You can gain more information about the last past of your question by knowing about the lifespan of Rupa Kalapa.

There is case where one develops perception of death, in short;

Life is uncertain death is certain I may die in 100 years... in 1 year... in a month... in a week... in a day... in an hour... on the in or out breath.

The dying on the in & out breath is the one bringing about most urgency.

Furthermore before going to sleep and after waking one contemplates;

In many ways can death come to me, proceeding to contemplate causes

having contemplated the various causes, one reviews;

Are there any bad qualities in the mind that would lead to my detriment if i was to die during the night or to die during the day?

If there are such bad qualities one resolves on working to elimimate them asap.

This would be kind of planning for future.

Refer to maranasati sutta 1&2. Thereis also a Sutta where the Buddha talks about how short the human life is in terms of how many seasons, days, nights and meals, it is a good one. Don't recall the title of the latter.

The way i do it is by starting with the general shortness of human life and then narrow it down to in and out breath. I also do the other morning/evening contemplation and the reviewing of mind.

One who develops perception of death is getting a lot of motivation & urgency to train and the priorities are straightened up.

It is also tied up with perception of impermanence and parting. These also have good results as they establish these inclinations in mind, distancing from the perception of attractiveness and reliability.

I usually get an affinity for that theme and am not much distracted. If i am distracted i change to something that i think will calm it down.

In general, planning is willful thinking and willful thinking is not meditation.

This means that you CAN plan, but that is not true meditation.

"Meditating on death" actually means "thinking deeply on death". That is reflection, not meditation.

It can be useful, but thinking, pondering, reflecting is not meditating.

Not sure where the quote is from, but the instant death that buddha talks about seems to regard the death of the self. He is talking about anicca and anatta, in other words.

One can of course plan for the future - or ones livelyhood - for instance, but there is a difference whether one plans with attachment or whether one plans with reminiscence of impermanence.