How do I fix "Error opening terminal: unknown." on Ubuntu server?

Try to run /bin/bash, I think it will allocate pseudo tty

Also try: TERM=linux then run nano

Webmin terminal is not interactive yet. In-fact, it's a command line interface.

You can read more about it, we discussed it quite a lot.

It's in our todo to make it interactive.

I encountered this problem when I was trying to edit files inside the initramfs. This was the only thread I found so instead of hunting for another fix, I wrote a python script to make a simple text editor that works inside the initramfs (and other feature poor terminals)

It's pretty simple, only displays one line at a time so you press up and down to change lines and left and right to move the cursor and enter to save. Nothing fancy, but it seems to work okay for quick edits.

It requires only the readchar module: python3 -m pip install readchar

#!/usr/bin/python3
#Edit a text file inside the initramfs (when other text editors don't work)
'''Compile with: 
libgcc=$(find /lib -name libgcc_s.so.1 | head -n 1)
libutil=$(ldd /usr/bin/python3 | grep libutil | cut -d ' ' -f 3)
pyinstaller --onefile editfile.py  --add-data="$libgcc:." --add-data="$libutil:." --hidden-import readchar
'''
import shutil, sys, readchar


'''
Allow user to edit a line of text complete with support for line wraps and a cursor | you can move back and forth with the arrow keys.
lines = initial text supplied to edit
prompt= Decoration presented before the text (not editable and not returned)
'''
def text_editor(lines=[], prompt=''):

    term_width = shutil.get_terminal_size()[0] - 1
    line_num = 0
    if type(lines) in (list, tuple):
        multiline=True
    else:
        multiline=False
        lines=[lines]


    text = list(lines[line_num])
    ptr = len(text)
    prompt = list(prompt)
    space = [' ']

    c = 0
    while True:
        if ptr and ptr > len(text):
            ptr = len(text)


        copy = text.copy()
        if ptr < len(text):
            copy.insert(ptr,'|')
        copy = list(str(line_num)) + space + prompt + copy

        #Line wraps support:
        if len(copy) > term_width:
            cut = len(copy) + 3 - term_width
            if ptr > len(copy) / 2:
                copy = ['<']*3 + copy[cut:]
            else:
                copy = copy[:-cut] + ['>']*3 

        print('\r'*term_width+''.join(copy), end=' '*(term_width-len(copy)), flush=True)

        if c in (53,54):
            #Page up/down bug
            c = readchar.readkey()
            if c == '~':
                continue
        else:
            c = readchar.readkey()  


        if len(c) > 1:
            #Control Character
            c = ord(c[-1])

            #Save current line on line change
            if c in (53, 54, 65, 66):
                lines[line_num] = ''.join(text)

            if c == 65:     #Up
                line_num -= 1
            elif c == 66:   #Down
                line_num += 1
            elif c == 68:   #Left
                ptr -= 1
            elif c == 67:   #Right
                ptr += 1
            elif c == 54:   #PgDn
                line_num += 10
            elif c == 53:   #PgUp
                line_num -= 10
            elif c == 70:   #End
                ptr = len(text)
            elif c == 72:   #Home
                ptr = 0
            else:
                print("\nUnknown control character:", c)
                print("Press ctrl-c to quit.")
                continue
            if ptr < 0:
                ptr = 0
            if ptr > len(text):
                ptr = len(text)

            #Check if line changed
            if c in (53, 54, 65, 66):
                if multiline == False:
                    line_num = 0
                    continue
                if line_num < 0:
                    line_num = 0
                while line_num > len(lines) - 1:
                    lines.append('')
                text = list(lines[line_num])


        else:

            num = ord(c)
            if num in (13, 10): #Enter
                print()
                lines[line_num] = ''.join(text)
                if multiline:
                    return lines
                else:
                    return lines[0]
            elif num == 127:        #Backspace
                if text:
                    text.pop(ptr-1)
                    ptr -=1
            elif num == 3:          #Ctrl-C 
                sys.exit(1)
            else:
                text.insert(ptr, c)
                ptr += 1

#Main
if len(sys.argv) == 1:
    print("Usage: ./editfile <filename>")
    sys.exit(1)

f = open(sys.argv[1], 'r')
strings = f.read().split('\n')
f.close()
strings = text_editor(strings)

#Trim empty lines on end
for x in range(len(strings) -1,0, -1):
    if len(strings[x]):
        break
    else:
        strings.pop(-1)     


f = open(sys.argv[1], 'w')
f.write('\n'.join(strings)+'\n')

I had the same problem and it was solved in the following way:

sudo apt-get install rxvt-unicode