The XML snippet you pasted does not conform to the query you have, it's missing the <Group> element you're looking for.
Either way, you'll need to
- have a correct namespace map (dict) – you currently have a set with one entry
- need to separate the namespace alias with a real colon :, not a fullwidth colon:
- have the namespace on each element of the query, as well as the Value subnode query.
I chose r (short for "report") as the alias for urn:crystal-reports:schemas:report-detail here. If you don't want to use aliases, you can also use the longhand syntax {urn:crystal-reports:schemas:report-detail}Group, etc., in which case you don't need the namespace map.
All that fixed, we get something like 
import xml.etree.ElementTree as et
data = """<?xml version="1.0"?>
<Report xmlns="urn:crystal-reports:schemas:report-detail" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="urn:crystal-reports:schemas:report-detail http://www.businessobjects.com/products/xml/CR2008Schema.xsd">
  <Group>
      <Details Level="1">
        <Field Name="ReportNo"><Value>90</Value></Field>
        <Field Name="Other"><Value>644</Value></Field>
      </Details>
  </Group>
</Report>
"""
nsmap = {"r": "urn:crystal-reports:schemas:report-detail"}
xroot = et.XML(data)  # could read from file here
for node in xroot.findall("r:Group/r:Details/r:Field", nsmap):
    name = node.attrib.get("Name")
    value = node.find("r:Value", nsmap).text
    print(name, value)
The output here is
ReportNo 90
Other 644
– plugging it into a dataframe is left as an exercise to the reader.