Trim in SSRS expression

TRIM removes whitespace. You want a SUBSTRING of the text.

Try this

declare @text varchar(50) = 'AM.JUBAIL.PRE-FINISHING-LINE.PFBSTR.PFBST1'
select reverse(left(reverse(@text), charindex('.',reverse(@text)) - 1))

Here is the reference material
LEFT (Transact-SQL) | Microsoft Docs[^]
REVERSE (Transact-SQL) | Microsoft Docs[^]
CHARINDEX (Transact-SQL) | Microsoft Docs[^]

If you look at the results of these queries you can see more easily what is going on ...

select reverse(@text)
select charindex('.',reverse(@text))
select left(reverse(@text), charindex('.',reverse(@text)) - 1)

Results ...

1TSBFP.RTSBFP.ENIL-GNIHSINIF-ERP.LIABUJ.MA
7
1TSBFP

[EDIT] Apologies - this was quite clearly tagged SSRS and I provided a SQL solution. Try the following instead (caveat - untested)

= Mid(Fields!LastEquipment.Value, InstrRev(Fields!LastEquipment.Value, ".") + 1, Length(Fields!LastEquipment.Value))

=Fields!LastEquipment.Value