Hi @Alex_Cristian  there’s a couple of typos in the formula that are stopping it from being saved in the field:
 There’s an additional closing bracket at the end of the formula which should be removed
 There’s a couple of instances of “Nr. Of Payments” missing its first curly bracket (lines 2 and 3).
 It is possible that the formatted quotes are also a problem but that could be the forum site doing this formatting and not what was entered into AT.
However, fixing all of these won’t fix your formula  there are a couple of logic errors to contend with too.

{Nr. Of Payments} = '2 Payments'
appears in all 3 IF statements which makes it redundant and not required (unless there could be other values in this field)
 On the assumption that you don’t need
{Nr. Of Payments} = '2 Payments'
, then you have the set of possible values of the other two fields to consider to determine the status:
Both checked, both unchecked, deposit checked/final not, deposit not checked/final checked. Some of these may be “error” states, e.g. deposit not checked, final checked, but they should still be account for.
The reason for noting these states is that your second IF statement:
IF(AND({Nr. Of Payments} =“2 Payments”, {Deposit Made}=TRUE()),“1 Payment Left”,
does not take account of the final payment status and so you will end up with “1 Payment Left” displaying whether final payment is checked or not. So, the answer here is to decide what all the combinations of states mean  perhaps something like this:
This would lead you to a formula of:
IF(
AND(NOT({Deposit Made}), NOT({Final Payment})),
'2 payments left',
IF(
AND({Deposit Made}, NOT({Final Payment})),
'1 payment left',
IF(AND(NOT({Deposit Made}), {Final Payment}),
'ERROR!',
'Paid in full'
)
)
)
Note: I prefer to use:
IF({Deposit Made}, ....)
rather than:
IF({Deposit Made}=TRUE(), ....)
Hope this helps!
JB