In case it didn’t help, proceed to the recommendation below.
Edit custom field contexts
In order to use a field for estimation, it must be configured in Contexts for projects and issue types you want to estimate.
Select Jira Settings (gear icon from the top right) > Issues.
Under FIELDS, click Custom fields.
Find the custom field you want to configure and click > Contexts and default value. Alternatively, you can click a field's screens or contexts link and then click Contexts > Create, edit, or delete contexts.
Locate the context you want to change and click Edit Configuration.
In the applicable issue types section, select or unselect the relevant issue types.
In the applicable contexts section, select or unselect the relevant projects.
Select Modify.
Edit issue layout
If you are using New Issue layout, navigate to Project Settings → Issue layout and make sure that the estimation field is showed by default
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