How much Free storage space do you have on this drive? Ignore the "Available" storage value since it is very misleading. You will need to look in Disk Utility for the Free space value.
How much storage does this other user account use?
If you are retaining the user account, then it will require at least the same amount of storage space as that user account uses while macOS creates an archive bundle for all the files within that user account. In fact you will probably need even more Free storage. Once macOS has finished archiving the data of the deleted user account, then some of that storage space will once again be returned for use with macOS.
FYI, you always need to have at least 20GB+ of Free storage space at all times for the normal operation of macOS. For an operation like deleting & archiving a user account, then you will need at least an extra amount equal to the size of the account being deleted and at least another 20GB+ for normal macOS operations.
For example, if the size of the user account being deleted is 30GB in size, then you will need at least 80GB+ of Free storage space (possibly even more depending on whether macOS also compresses the data....if the data is compressed you would need to double the amount of storage.
80GB+ = 30GB + 30GB + 20GB+
If macOS compresses the data in the archive, then temporarily you would need 110GB+ (30 + 30 + 30 + 20)
If you have a sufficient amount of Free storage space, then you may want to try deleting the user account while booted into Safe Mode just in case you have some third party installed software that is interfering with the normal operation of macOS.
Last option would be to delete the user account and also delete its data.
It is very unfortunate macOS does not provide more descriptive error messages.