The crucial thing is to understand what is causing the error.

Examples of possibilities:

1. The subform has no records and new records cannot be added. In this case,
the subform's detail section goes completely blank, and any reference to the
non-existent text box causes an error.
The solution would be something like this:
=IIf([MySub].[Form].[RecordsetClone].[RecordCount] = 0, 0,
Nz([MySub].[Form].[Text0],0))

2. Malformed statement.
If your text box contains something like this:
   =DLookup("MyField", "MyTable", "[Field2] = " & [Field2])
you are asking Access to concatenate the value of Field2 into the 3rd
argument. But if Field2 is null (e.g. at a new record), the 3rd argument
becomes just:
   [Field2] =
which is clearly mal-formed. To avoid this, add Nz(), e.g.:
   =DLookup("MyField", "MyTable", "[Field2] = " & Nz([Field2],0))

3. Other error
A simple statment like:
   =[Field1] / [Field2]
generates an error when Field2 is zero. (Divide by zero error).
Solution:
   =IIf([Field2]=0, 0, [Field1] / [Field2])
Again, there are heaps of way to generate an error: even setting a control's
Format property to Short Date and then entering any text that is not a date.

So, figure out what is causing the error, and you know how to solve it.