You need to wrap it inside the <Field>
tag as below in the element.xml file.
<Field ID="{060E50AC-E9C1-4D3C-B1F9-DE0BCAC300F6}"
Name="EmailId"
DisplayName="Email Id"
Type="Text"
Description ="Some desc"
Required="TRUE"
MaxLength="50"
Group="SPFx Columns"></Field>
So, your full XML would be somewhat as below:
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<Field ID="{060E50AC-E9C1-4D3C-B1F9-DE0BCAC300F6}"
Name="EmailId"
DisplayName="Email Id"
Type="Text"
Description ="Some desc."
Required="TRUE"
MaxLength="50"
Group="SPFx Columns" >
<Field ID="{943E7530-5E2B-4C02-8259-CCD93A9ECB18}"
Name="CustomChoice"
DisplayName="Choice Field"
Type="Choice"
Required="FALSE"
Group="SPFx Columns">
<CHOICES>
<CHOICE>Blue</CHOICE>
<CHOICE>Green</CHOICE>
<CHOICE>Green</CHOICE>
</CHOICES>
</Field>
<ContentType ID="0x010042D0C1C200A14B6887742B6344675C8B"
Name="My Content Type"
Group="SPFx Content Types"
Description="Sample content types from web part solution">
<FieldRefs>
<FieldRef ID="{060E50AC-E9C1-4D3C-B1F9-DE0BCAC300F6}" />
<FieldRef ID="{943E7530-5E2B-4C02-8259-CCD93A9ECB18}" />
</FieldRefs>
</ContentType>
<ListInstance
FeatureId="00bfea71-de22-43b2-a848-c05709900100"
Title="TestSPFxList3"
Description="New SP List"
TemplateType="100"
Url="Lists/TestSPFxList3">
</ListInstance>
</Elements>
Reference- Provision SP assets from package