I am trying to install ubuntu alngside windows 10 on an asus laptop. However, when I come to the partition screen the ubuntu installer only detects of the whole 900gb disk only the 400 mb partition on which windows is installed and does not see the 300gb free space I have allocated for the installation.
I have a BIOS system and fast boot and secure boot are disabled. I am trying to install ubuntu 20 lts.
The first thing that comes to my mind is that maybe you have already 4 primary partition created. You can only have a max of 4 primary. If you need more partitions you need to use an extended partition.
If you boot with Ubuntu Live, can you execute from the terminal:
cat /proc/partitions
Did you resize your partitions?. That does not guarantee you to see all the space resized as available for a partition.
Windows does not fit in 400 MB, you wanted to say 400 GB or it can be a partition for manufacturer Windows installer.
Update: After seeing the images (sorry if you linked them in the first place I didn't click on those links) I can tell you why I think this happens. If you look at your windows partition manager screenshot you have the small partition, the windows partition, empty space, and space in use at the end (in blue). I understand that you rearranged partitions, but in this process something didn't like to Ubuntu installer.
I would like to know what is the space at the end. You should be able to see the partition table and if it's healthy to create a partition by booting Ubuntu Live and from a Terminal selecting:
fdisk /dev/sda
You can use h for help p for printing the partitions n for creating a new one. If you create a new one fdisk will offer you only the space that it has available. Resizing and moving partitions and creating new after is dangerous, please do a backup before continuing if you didn't.
So in your case you can't see the free space so you can try this:-
1st - First allocate the free space by right clicking the free space and then merge it to your main partition where the windows is installed .
2nd - Now select ubuntu along side windows and select the partition where the windows is installed and you will see that you can partition the main drive there as you choice by dragging the line between the screen just like this 
((Image credit :- Linuxconfig.org)) OR if you care about your data and want to take safer way the make the unallocated space a named drive such as 'D Drive' and then select this drive during the installation of Ubuntu and do the same as shown in the image. 3rd - Now the space you didn't gave to Ubuntu will be left in D Drive or the name you gave. complete the installation. After you come in windows and open disk management you will see that the drive you named will have the space you left and a drive with no name, there the Ubuntu is installed.
This is most likely due to a "hidden" SATA setting in the BIOS. The solution is a bit tricky, but this might just work for you:
First you will need to enable "Safe Mode".
msconfig[Apply] and [OK]
Note: Even with "Network" selected, some people do not have a working WiFi connection in Safe Mode. Be sure to have a wired connection ready just in case.
Acer seems to have some "hidden" features for many of their newer machines. SATA mode, which you need, is one of them.
You will see "Safe Mode" messages in all four corners of your screen. This is fine, as you'll change this later. When you boot into Windows, you will need to install the AHCI driver so that the computer can properly read the storage device that you just put into AHCI mode.
This is how you do it:
With all the Windows stuff out of the way, you can now reboot the machine with the Ubuntu Live USB device plugged in, boot into "Try Ubuntu" (or "Install Ubuntu"), select the 293GB partition that you have set aside, and install the operating system.
Hope this gives you what you're looking for