I was able to fix part of the error by by removing !=BLANK()
from your second IF function. When checking to see if a field is not empty, it’s preferable (and more reliable) to use the shortcut format:
IF({date manuelle prévue}, ...)
…which returns True if {date manuelle prévue}
contains anything.
Doing that got rid of the error, but left the resulting date looking like this:
2019-04-09T00:00:00.000Z
I then wrapped a similar DATETIME_FORMAT around the DATEADD, which gave me this:
9/4/2019
Here’s the full formula:
IF(
relance="pas d’action"
,"ok"
,IF(
{date manuelle prévue},
DATETIME_FORMAT(
{date manuelle prévue},
'D/M/YYYY'
),
DATETIME_FORMAT(
DATEADD(
{date de l’échange},8,'days'
),
'D/M/YYYY'
)
)
)
BTW, you said in your comments that you wanted to add 12 days, but had 8 in the formula, so I left it at 8.