On the left wall of my room, there are lots of postcards which Ithat I have bought them during trips.

it's just a bit more clear and concise!

On the left wall of my room, there are lots of postcards whichthat I bought them during trips.

The pictures on the postcards are about sceneries of Tokyo, Guam, Bali, Warsaw and so on.

When I visit a foreign country, I always buy post cards whichthat describe the country or the city well.

With the postcards on the wall, I can remember my memories when traveling.

Your sentence is correct, but this sounds a little more natural:

- With the postcards on the wall, I can recollect my memories from when I traveled.

- With the postcards on the wall, I can recall my memories from when I traveled.

Unfortunately, now, I can't collect the postcards because of the pandemic.

Your sentence is correct, but this sounds a little more natural: Unfortunately, I now can't collect the postcards because of the pandemic.

On the left wall of my room, there are lots of postcards, which I bought them during trips.

The pictures on the postcards are aboutof sceneries ofin Tokyo, Guam, Bali, Warsaw and so on.

When I visit a foreign country, I always buy post cards which describerepresent the country or the city well.

With the postcards on the wall, I can rememberflect on my memories whenfrom traveling.

"when traveling" would imply that the wall of postcards go with you on your travels. Also; I replaced "remember" with "reflect on" to reduce redundancy, though this is optional. "Reminisce" is also a good word here.

Unfortunately, now, I can't collect the postcards because of the pandemic.

We only need a definite article (the) in front of postcards if you were referring to a specific one. Since you seem to be referring to postcards in general, we can omit "the"

On the left side wall of my room, there are lots of postcards whichhanging up that I bought them during my trips abroad.

The pictures on the postcards are about scenerilandscapes of Tokyo, Guam, Bali, Warsaw and so onmore.

When I visit a foreign country, I always buy post cards which describepict the country or the city well.

When you are talking about an image, it is better to use the word "depict" rather than "describe."

With these postcards on the wall, I can remember my memories whenexperiences from my travelings.

Unfortunately, now, I can't collect the postcardsadd to my postcard collection because of the pandemic.

On the left wall of my room, there are lots of postcards whichthat I bought them duringon my trips.

The pictures on the postcards are about sceneries of Tokyo, Guam, Bali, Warsaw and so onmore.

When I visit a foreign country, I always buy post cards which describerepresent the country or the city well.

With the postcards on the wall, I can remember my memories whenfrom my travelings.

Unfortunately, now, I can't collect the postcards right now because of the pandemic.